Is the classifying space a fully faithful functor?The fundamental group functor is not full. Counterexample? Subcategories with full restriction?Is the homotopy category cartesian closed?Example of a cogroup in $mathsfhTop_bullet$ which is not a suspensionDoes a group homomorphism up to homotopy induce a map between classifying spaces?Classifying Space for What is the Infinite Unitary Group?Calculation of the first Chern class of the canonical line bundle over $mathbbCP^n$Universal coverings and fully faithful fiber functors?References for classifying spaces and cohomology of classifying spacesStrong homotopy equivalence on classifying spaces via weak homotopy equivalence on classifying topoiWhy is it interesting that the fundamental group induces a functor?
Is a bound state a stationary state?
Create all possible words using a set or letters
What should you do if you miss a job interview (deliberately)?
How much character growth crosses the line into breaking the character
Should I outline or discovery write my stories?
Does a 'pending' US visa application constitute a denial?
Is it better practice to read straight from sheet music rather than memorize it?
What prevents the use of a multi-segment ILS for non-straight approaches?
Store Credit Card Information in Password Manager?
Melting point of aspirin, contradicting sources
How do I color the graph in datavisualization?
Can I sign legal documents with a smiley face?
Why is so much work done on numerical verification of the Riemann Hypothesis?
Is it safe to use olive oil to clean the ear wax?
Not using 's' for he/she/it
Why do compilers behave differently when static_cast(ing) a function to void*?
What does chmod -u do?
Is it possible to have a strip of cold climate in the middle of a planet?
Fear of getting stuck on one programming language / technology that is not used in my country
Argument list too long when zipping large list of certain files in a folder
Count the occurrence of each unique word in the file
Are the IPv6 address space and IPv4 address space completely disjoint?
Is there a name for this algorithm to calculate the concentration of a mixture of two solutions containing the same solute?
Open a doc from terminal, but not by its name
Is the classifying space a fully faithful functor?
The fundamental group functor is not full. Counterexample? Subcategories with full restriction?Is the homotopy category cartesian closed?Example of a cogroup in $mathsfhTop_bullet$ which is not a suspensionDoes a group homomorphism up to homotopy induce a map between classifying spaces?Classifying Space for What is the Infinite Unitary Group?Calculation of the first Chern class of the canonical line bundle over $mathbbCP^n$Universal coverings and fully faithful fiber functors?References for classifying spaces and cohomology of classifying spacesStrong homotopy equivalence on classifying spaces via weak homotopy equivalence on classifying topoiWhy is it interesting that the fundamental group induces a functor?
$begingroup$
Given a topological group $G$, we can form its classifying space $BG$; suppose we have chosen some specific construction, say the bar construction. $B$ is a functor - given any homomorphism $G to H$, it induces a continuous map $BG to BH$.
For discrete groups $G$ and $H$, these are all the maps $BG to BH$ up to (basepointed) homotopy; one can prove more or less by hand that any map between Eilenberg MacLane spaces $K(G,n) to K(H,n)$ are classified up to homotopy by the homomorphisms $G to H$ they induce on $pi_n$, and all homomorphisms are induced this way.
Is this true for arbitrary topological groups $G$ and $H$? Do different homomorphisms $G to H$ induce homotopically distinct maps $BG to BH$, and are all maps $BG to BH$ homotopic to a map induced by a homomorphism? Put another way, is the functor $B: mathsfTopGrp to mathsfhTop_*$ fully faithful?
Because maps into $BG$ represent $G$-bundles, one reason to care is that this would tell us that the only natural way to go from $G$-bundles to $H$-bundles is via a homomorphism $G to H$; one might expect this to begin with. If one cares about classifying homotopy classes of maps between certain spaces, it immediately tells you what the maps are between the (infinite) lens spaces and $BbbCP^infty$.
Edit: as pointed out by Zhen Lin below, this is trivially false. So let's make some minor modifications to avoid his counterexample: either restrict $G$ and $H$ to be compact (even Lie, maybe? this is the case I care about) groups, or alternately pass to the homotopy category of topological groups, where my morphisms are homotopy (through homomorphisms) classes of continuous homomorphisms; these two ideas should be related by the Iwasawa decomposition, by which a locally compact topological group deformation retracts onto its maximal compact subgroup.
Are homotopy classes of continuous maps $BG to BH$ the same as homotopy classes of homomorphisms $G to H$?
algebraic-topology classifying-spaces
$endgroup$
add a comment |
$begingroup$
Given a topological group $G$, we can form its classifying space $BG$; suppose we have chosen some specific construction, say the bar construction. $B$ is a functor - given any homomorphism $G to H$, it induces a continuous map $BG to BH$.
For discrete groups $G$ and $H$, these are all the maps $BG to BH$ up to (basepointed) homotopy; one can prove more or less by hand that any map between Eilenberg MacLane spaces $K(G,n) to K(H,n)$ are classified up to homotopy by the homomorphisms $G to H$ they induce on $pi_n$, and all homomorphisms are induced this way.
Is this true for arbitrary topological groups $G$ and $H$? Do different homomorphisms $G to H$ induce homotopically distinct maps $BG to BH$, and are all maps $BG to BH$ homotopic to a map induced by a homomorphism? Put another way, is the functor $B: mathsfTopGrp to mathsfhTop_*$ fully faithful?
Because maps into $BG$ represent $G$-bundles, one reason to care is that this would tell us that the only natural way to go from $G$-bundles to $H$-bundles is via a homomorphism $G to H$; one might expect this to begin with. If one cares about classifying homotopy classes of maps between certain spaces, it immediately tells you what the maps are between the (infinite) lens spaces and $BbbCP^infty$.
Edit: as pointed out by Zhen Lin below, this is trivially false. So let's make some minor modifications to avoid his counterexample: either restrict $G$ and $H$ to be compact (even Lie, maybe? this is the case I care about) groups, or alternately pass to the homotopy category of topological groups, where my morphisms are homotopy (through homomorphisms) classes of continuous homomorphisms; these two ideas should be related by the Iwasawa decomposition, by which a locally compact topological group deformation retracts onto its maximal compact subgroup.
Are homotopy classes of continuous maps $BG to BH$ the same as homotopy classes of homomorphisms $G to H$?
algebraic-topology classifying-spaces
$endgroup$
2
$begingroup$
It's not even true for discrete groups unless you go to the pointed homotopy category.
$endgroup$
– Zhen Lin
Sep 30 '15 at 18:52
$begingroup$
@ZhenLin Fair enough, that's what I meant. Let me edit.
$endgroup$
– user98602
Sep 30 '15 at 18:53
add a comment |
$begingroup$
Given a topological group $G$, we can form its classifying space $BG$; suppose we have chosen some specific construction, say the bar construction. $B$ is a functor - given any homomorphism $G to H$, it induces a continuous map $BG to BH$.
For discrete groups $G$ and $H$, these are all the maps $BG to BH$ up to (basepointed) homotopy; one can prove more or less by hand that any map between Eilenberg MacLane spaces $K(G,n) to K(H,n)$ are classified up to homotopy by the homomorphisms $G to H$ they induce on $pi_n$, and all homomorphisms are induced this way.
Is this true for arbitrary topological groups $G$ and $H$? Do different homomorphisms $G to H$ induce homotopically distinct maps $BG to BH$, and are all maps $BG to BH$ homotopic to a map induced by a homomorphism? Put another way, is the functor $B: mathsfTopGrp to mathsfhTop_*$ fully faithful?
Because maps into $BG$ represent $G$-bundles, one reason to care is that this would tell us that the only natural way to go from $G$-bundles to $H$-bundles is via a homomorphism $G to H$; one might expect this to begin with. If one cares about classifying homotopy classes of maps between certain spaces, it immediately tells you what the maps are between the (infinite) lens spaces and $BbbCP^infty$.
Edit: as pointed out by Zhen Lin below, this is trivially false. So let's make some minor modifications to avoid his counterexample: either restrict $G$ and $H$ to be compact (even Lie, maybe? this is the case I care about) groups, or alternately pass to the homotopy category of topological groups, where my morphisms are homotopy (through homomorphisms) classes of continuous homomorphisms; these two ideas should be related by the Iwasawa decomposition, by which a locally compact topological group deformation retracts onto its maximal compact subgroup.
Are homotopy classes of continuous maps $BG to BH$ the same as homotopy classes of homomorphisms $G to H$?
algebraic-topology classifying-spaces
$endgroup$
Given a topological group $G$, we can form its classifying space $BG$; suppose we have chosen some specific construction, say the bar construction. $B$ is a functor - given any homomorphism $G to H$, it induces a continuous map $BG to BH$.
For discrete groups $G$ and $H$, these are all the maps $BG to BH$ up to (basepointed) homotopy; one can prove more or less by hand that any map between Eilenberg MacLane spaces $K(G,n) to K(H,n)$ are classified up to homotopy by the homomorphisms $G to H$ they induce on $pi_n$, and all homomorphisms are induced this way.
Is this true for arbitrary topological groups $G$ and $H$? Do different homomorphisms $G to H$ induce homotopically distinct maps $BG to BH$, and are all maps $BG to BH$ homotopic to a map induced by a homomorphism? Put another way, is the functor $B: mathsfTopGrp to mathsfhTop_*$ fully faithful?
Because maps into $BG$ represent $G$-bundles, one reason to care is that this would tell us that the only natural way to go from $G$-bundles to $H$-bundles is via a homomorphism $G to H$; one might expect this to begin with. If one cares about classifying homotopy classes of maps between certain spaces, it immediately tells you what the maps are between the (infinite) lens spaces and $BbbCP^infty$.
Edit: as pointed out by Zhen Lin below, this is trivially false. So let's make some minor modifications to avoid his counterexample: either restrict $G$ and $H$ to be compact (even Lie, maybe? this is the case I care about) groups, or alternately pass to the homotopy category of topological groups, where my morphisms are homotopy (through homomorphisms) classes of continuous homomorphisms; these two ideas should be related by the Iwasawa decomposition, by which a locally compact topological group deformation retracts onto its maximal compact subgroup.
Are homotopy classes of continuous maps $BG to BH$ the same as homotopy classes of homomorphisms $G to H$?
algebraic-topology classifying-spaces
algebraic-topology classifying-spaces
edited Sep 30 '15 at 19:50
asked Sep 30 '15 at 18:51
user98602
2
$begingroup$
It's not even true for discrete groups unless you go to the pointed homotopy category.
$endgroup$
– Zhen Lin
Sep 30 '15 at 18:52
$begingroup$
@ZhenLin Fair enough, that's what I meant. Let me edit.
$endgroup$
– user98602
Sep 30 '15 at 18:53
add a comment |
2
$begingroup$
It's not even true for discrete groups unless you go to the pointed homotopy category.
$endgroup$
– Zhen Lin
Sep 30 '15 at 18:52
$begingroup$
@ZhenLin Fair enough, that's what I meant. Let me edit.
$endgroup$
– user98602
Sep 30 '15 at 18:53
2
2
$begingroup$
It's not even true for discrete groups unless you go to the pointed homotopy category.
$endgroup$
– Zhen Lin
Sep 30 '15 at 18:52
$begingroup$
It's not even true for discrete groups unless you go to the pointed homotopy category.
$endgroup$
– Zhen Lin
Sep 30 '15 at 18:52
$begingroup$
@ZhenLin Fair enough, that's what I meant. Let me edit.
$endgroup$
– user98602
Sep 30 '15 at 18:53
$begingroup$
@ZhenLin Fair enough, that's what I meant. Let me edit.
$endgroup$
– user98602
Sep 30 '15 at 18:53
add a comment |
3 Answers
3
active
oldest
votes
$begingroup$
The correct and invariant statement is that the classifying space functor is an equivalence of $(infty, 1)$-categories between grouplike $E_1$ spaces (this is a homotopically invariant version of "topological group") and pointed connected spaces (and every time I say "spaces" I mean "weak homotopy types"). I believe it's also true that every grouplike $E_1$ space can be modeled by a topological group.
Trying to strictify this equivalence to a statement about homomorphisms beween topological groups is hopeless in general. The maps corresponding to maps $BG to BH$ are maps $G to H$ that respect the group operation up to coherent homotopy, so there's no reason to expect the classifying space functor, as a functor on topological groups and taking values in homotopy categories, to be full.
Explicitly, take $G$ to be discrete and take $H = S^1$. Then $[BG, BH] cong H^2(G, mathbbZ)$, which by the universal coefficient theorem has two summands, namely
$$textExt^1(H_1(G, mathbbZ), mathbbZ)$$
and $textHom(H_2(G, mathbbZ), mathbbZ)$. Group homomorphisms $G to S^1$ only give you terms in the first summand, via the Ext long exact sequence coming from the short exact sequence
$$0 to mathbbZ to mathbbR to S^1 to 0$$
giving a long exact sequence with connecting map $textHom(H_1(G), S^1) to textExt^1(H_1(G, mathbbZ), mathbbZ)$.
The second summand $textHom(H_2(G, mathbbZ), mathbbZ)$ is related to the nontrivial structure implied by respecting group operations up to coherent homotopy. (This means that $f(g_1 g_2)$ isn't equal to $f(g_1) f(g_2)$, but that there's a homotopy relating them which itself is required to satisfy certain axioms. This homotopy is an embellished version of a $2$-cocycle.)
An earlier version of this answer claimed a counterexample for compact Lie groups (the above example with $G$ finite) that doesn't work; thanks to Omar Antolín-Camarena for pointing this out. I don't know enough about e.g. the homotopy theory of $BSU(2)$ to write down a counterexample for compact Lie groups. I might start by looking at $G = S^1, H = SU(2)$.
$endgroup$
1
$begingroup$
Homotopy clases of continuos group homomorphisms are the wrong thing to look at only if they are not what you have to look at. That sort of claim is only true in situations where one has nothing to do but deontics! :-)
$endgroup$
– Mariano Suárez-Álvarez
Oct 1 '15 at 2:58
$begingroup$
@MarianoSuárez-Alvarez: I'm not sure I agree. In any case, I learned something from this answer.
$endgroup$
– user98602
Oct 1 '15 at 3:32
1
$begingroup$
@Mike: in general, when $H = U(n)$ this issue is closely related to the difference between Borel-equivariant and genuinely equivariant K-theory (of a point); Borel $G$-equivariant K-theory of a point is the K-theory of $BG$, while genuinely $G$-equivariant K-theory of a point is the representation theory of $G$. These two differ but are related by the Atiyah-Segal completion theorem.
$endgroup$
– Qiaochu Yuan
Oct 1 '15 at 16:52
1
$begingroup$
Isn't the other summand in $H^2(G,mathbbZ)$ given by $mathrmHom(H_2(G,mathbbZ), mathbbZ)$, which vanishes because for a finite group $G$, $H_2(G, mathbbZ)$ is finite too?
$endgroup$
– Omar Antolín-Camarena
Apr 22 '16 at 1:33
1
$begingroup$
In Maps of BZ/p to BG, Dwyer and Wilkerson show that the map Hom(G, SU(2))/conjugation --> [BG, BSU(2)] (that's homotopy classes of maps, not of pointed maps, which is why the domain is conjugation classes of group homomorphisms) is not surjective if G is the symmetric group on 3 letters and also that it is not injective if G is cyclic of order 15.
$endgroup$
– Omar Antolín-Camarena
Apr 22 '16 at 17:46
|
show 4 more comments
$begingroup$
The functor $B : mathbfTopGrp to operatornameHo mathbfTop_*$ is not fully faithful. Indeed, it does not even preserve non-isomorphy: the multiplicative group $mathbbR^times$ and the discrete group $mathbbZ / 2 mathbbZ$ have the same classifying space (namely, $mathbbR P^infty$), so if the functor were fully faithful, $mathbbR^times$ and $mathbbZ / 2 mathbbZ$ would have to be isomorphic as topological groups – but they are not.
$endgroup$
$begingroup$
How annoyingly trivial. Is this fixable by changing the former category or expanding the latter? (Perhaps by restricting to compact groups?)
$endgroup$
– user98602
Sep 30 '15 at 19:03
$begingroup$
Well, at minimum you should replace $mathbfTopGrp$ with $operatornameHo mathbfTopGrp$. But even then I suspect the answer is no.
$endgroup$
– Zhen Lin
Sep 30 '15 at 19:04
$begingroup$
I agree that is a better replacement than restricting to compact groups (though I don't know what the morphisms in your homotopy category are - homotopies through homomorphisms?) Please let me know if you find a contradiction to this statement.
$endgroup$
– user98602
Sep 30 '15 at 19:10
$begingroup$
Homotopies through homomorphisms is probably the simplest option. We would need to have induced homotopies of the morphisms of bar constructions.
$endgroup$
– Zhen Lin
Sep 30 '15 at 19:40
add a comment |
$begingroup$
I know this is an old+answered question, but I don't think the following result has been explicitly brought up:
Let $G$ be a Lie group, $gin G$, and let $c_gcolon Gto G$ be the automorphism given by $c_g(h) = ghg^-1$. Then the induced map $B c_gcolon BG to BG$ is homotopic to the identity map.
(see for example Ebert's Notes on index theory, lemma 9.2.2 on page 143)
Then any $(G, g)$ such that $c_g$ is not homotopic to the identity map (for example $G$ discrete and $g$ not in the centre) is a counter-example.
Based on Omar Antolín-Camarena's comment on Qiaochu Yuan's answer, it seems like quotienting out the inner automorphisms would not be enough to make it fully-faithful in general either.
$endgroup$
add a comment |
Your Answer
StackExchange.ifUsing("editor", function ()
return StackExchange.using("mathjaxEditing", function ()
StackExchange.MarkdownEditor.creationCallbacks.add(function (editor, postfix)
StackExchange.mathjaxEditing.prepareWmdForMathJax(editor, postfix, [["$", "$"], ["\\(","\\)"]]);
);
);
, "mathjax-editing");
StackExchange.ready(function()
var channelOptions =
tags: "".split(" "),
id: "69"
;
initTagRenderer("".split(" "), "".split(" "), channelOptions);
StackExchange.using("externalEditor", function()
// Have to fire editor after snippets, if snippets enabled
if (StackExchange.settings.snippets.snippetsEnabled)
StackExchange.using("snippets", function()
createEditor();
);
else
createEditor();
);
function createEditor()
StackExchange.prepareEditor(
heartbeatType: 'answer',
autoActivateHeartbeat: false,
convertImagesToLinks: true,
noModals: true,
showLowRepImageUploadWarning: true,
reputationToPostImages: 10,
bindNavPrevention: true,
postfix: "",
imageUploader:
brandingHtml: "Powered by u003ca class="icon-imgur-white" href="https://imgur.com/"u003eu003c/au003e",
contentPolicyHtml: "User contributions licensed under u003ca href="https://creativecommons.org/licenses/by-sa/3.0/"u003ecc by-sa 3.0 with attribution requiredu003c/au003e u003ca href="https://stackoverflow.com/legal/content-policy"u003e(content policy)u003c/au003e",
allowUrls: true
,
noCode: true, onDemand: true,
discardSelector: ".discard-answer"
,immediatelyShowMarkdownHelp:true
);
);
Sign up or log in
StackExchange.ready(function ()
StackExchange.helpers.onClickDraftSave('#login-link');
);
Sign up using Google
Sign up using Facebook
Sign up using Email and Password
Post as a guest
Required, but never shown
StackExchange.ready(
function ()
StackExchange.openid.initPostLogin('.new-post-login', 'https%3a%2f%2fmath.stackexchange.com%2fquestions%2f1458470%2fis-the-classifying-space-a-fully-faithful-functor%23new-answer', 'question_page');
);
Post as a guest
Required, but never shown
3 Answers
3
active
oldest
votes
3 Answers
3
active
oldest
votes
active
oldest
votes
active
oldest
votes
$begingroup$
The correct and invariant statement is that the classifying space functor is an equivalence of $(infty, 1)$-categories between grouplike $E_1$ spaces (this is a homotopically invariant version of "topological group") and pointed connected spaces (and every time I say "spaces" I mean "weak homotopy types"). I believe it's also true that every grouplike $E_1$ space can be modeled by a topological group.
Trying to strictify this equivalence to a statement about homomorphisms beween topological groups is hopeless in general. The maps corresponding to maps $BG to BH$ are maps $G to H$ that respect the group operation up to coherent homotopy, so there's no reason to expect the classifying space functor, as a functor on topological groups and taking values in homotopy categories, to be full.
Explicitly, take $G$ to be discrete and take $H = S^1$. Then $[BG, BH] cong H^2(G, mathbbZ)$, which by the universal coefficient theorem has two summands, namely
$$textExt^1(H_1(G, mathbbZ), mathbbZ)$$
and $textHom(H_2(G, mathbbZ), mathbbZ)$. Group homomorphisms $G to S^1$ only give you terms in the first summand, via the Ext long exact sequence coming from the short exact sequence
$$0 to mathbbZ to mathbbR to S^1 to 0$$
giving a long exact sequence with connecting map $textHom(H_1(G), S^1) to textExt^1(H_1(G, mathbbZ), mathbbZ)$.
The second summand $textHom(H_2(G, mathbbZ), mathbbZ)$ is related to the nontrivial structure implied by respecting group operations up to coherent homotopy. (This means that $f(g_1 g_2)$ isn't equal to $f(g_1) f(g_2)$, but that there's a homotopy relating them which itself is required to satisfy certain axioms. This homotopy is an embellished version of a $2$-cocycle.)
An earlier version of this answer claimed a counterexample for compact Lie groups (the above example with $G$ finite) that doesn't work; thanks to Omar Antolín-Camarena for pointing this out. I don't know enough about e.g. the homotopy theory of $BSU(2)$ to write down a counterexample for compact Lie groups. I might start by looking at $G = S^1, H = SU(2)$.
$endgroup$
1
$begingroup$
Homotopy clases of continuos group homomorphisms are the wrong thing to look at only if they are not what you have to look at. That sort of claim is only true in situations where one has nothing to do but deontics! :-)
$endgroup$
– Mariano Suárez-Álvarez
Oct 1 '15 at 2:58
$begingroup$
@MarianoSuárez-Alvarez: I'm not sure I agree. In any case, I learned something from this answer.
$endgroup$
– user98602
Oct 1 '15 at 3:32
1
$begingroup$
@Mike: in general, when $H = U(n)$ this issue is closely related to the difference between Borel-equivariant and genuinely equivariant K-theory (of a point); Borel $G$-equivariant K-theory of a point is the K-theory of $BG$, while genuinely $G$-equivariant K-theory of a point is the representation theory of $G$. These two differ but are related by the Atiyah-Segal completion theorem.
$endgroup$
– Qiaochu Yuan
Oct 1 '15 at 16:52
1
$begingroup$
Isn't the other summand in $H^2(G,mathbbZ)$ given by $mathrmHom(H_2(G,mathbbZ), mathbbZ)$, which vanishes because for a finite group $G$, $H_2(G, mathbbZ)$ is finite too?
$endgroup$
– Omar Antolín-Camarena
Apr 22 '16 at 1:33
1
$begingroup$
In Maps of BZ/p to BG, Dwyer and Wilkerson show that the map Hom(G, SU(2))/conjugation --> [BG, BSU(2)] (that's homotopy classes of maps, not of pointed maps, which is why the domain is conjugation classes of group homomorphisms) is not surjective if G is the symmetric group on 3 letters and also that it is not injective if G is cyclic of order 15.
$endgroup$
– Omar Antolín-Camarena
Apr 22 '16 at 17:46
|
show 4 more comments
$begingroup$
The correct and invariant statement is that the classifying space functor is an equivalence of $(infty, 1)$-categories between grouplike $E_1$ spaces (this is a homotopically invariant version of "topological group") and pointed connected spaces (and every time I say "spaces" I mean "weak homotopy types"). I believe it's also true that every grouplike $E_1$ space can be modeled by a topological group.
Trying to strictify this equivalence to a statement about homomorphisms beween topological groups is hopeless in general. The maps corresponding to maps $BG to BH$ are maps $G to H$ that respect the group operation up to coherent homotopy, so there's no reason to expect the classifying space functor, as a functor on topological groups and taking values in homotopy categories, to be full.
Explicitly, take $G$ to be discrete and take $H = S^1$. Then $[BG, BH] cong H^2(G, mathbbZ)$, which by the universal coefficient theorem has two summands, namely
$$textExt^1(H_1(G, mathbbZ), mathbbZ)$$
and $textHom(H_2(G, mathbbZ), mathbbZ)$. Group homomorphisms $G to S^1$ only give you terms in the first summand, via the Ext long exact sequence coming from the short exact sequence
$$0 to mathbbZ to mathbbR to S^1 to 0$$
giving a long exact sequence with connecting map $textHom(H_1(G), S^1) to textExt^1(H_1(G, mathbbZ), mathbbZ)$.
The second summand $textHom(H_2(G, mathbbZ), mathbbZ)$ is related to the nontrivial structure implied by respecting group operations up to coherent homotopy. (This means that $f(g_1 g_2)$ isn't equal to $f(g_1) f(g_2)$, but that there's a homotopy relating them which itself is required to satisfy certain axioms. This homotopy is an embellished version of a $2$-cocycle.)
An earlier version of this answer claimed a counterexample for compact Lie groups (the above example with $G$ finite) that doesn't work; thanks to Omar Antolín-Camarena for pointing this out. I don't know enough about e.g. the homotopy theory of $BSU(2)$ to write down a counterexample for compact Lie groups. I might start by looking at $G = S^1, H = SU(2)$.
$endgroup$
1
$begingroup$
Homotopy clases of continuos group homomorphisms are the wrong thing to look at only if they are not what you have to look at. That sort of claim is only true in situations where one has nothing to do but deontics! :-)
$endgroup$
– Mariano Suárez-Álvarez
Oct 1 '15 at 2:58
$begingroup$
@MarianoSuárez-Alvarez: I'm not sure I agree. In any case, I learned something from this answer.
$endgroup$
– user98602
Oct 1 '15 at 3:32
1
$begingroup$
@Mike: in general, when $H = U(n)$ this issue is closely related to the difference between Borel-equivariant and genuinely equivariant K-theory (of a point); Borel $G$-equivariant K-theory of a point is the K-theory of $BG$, while genuinely $G$-equivariant K-theory of a point is the representation theory of $G$. These two differ but are related by the Atiyah-Segal completion theorem.
$endgroup$
– Qiaochu Yuan
Oct 1 '15 at 16:52
1
$begingroup$
Isn't the other summand in $H^2(G,mathbbZ)$ given by $mathrmHom(H_2(G,mathbbZ), mathbbZ)$, which vanishes because for a finite group $G$, $H_2(G, mathbbZ)$ is finite too?
$endgroup$
– Omar Antolín-Camarena
Apr 22 '16 at 1:33
1
$begingroup$
In Maps of BZ/p to BG, Dwyer and Wilkerson show that the map Hom(G, SU(2))/conjugation --> [BG, BSU(2)] (that's homotopy classes of maps, not of pointed maps, which is why the domain is conjugation classes of group homomorphisms) is not surjective if G is the symmetric group on 3 letters and also that it is not injective if G is cyclic of order 15.
$endgroup$
– Omar Antolín-Camarena
Apr 22 '16 at 17:46
|
show 4 more comments
$begingroup$
The correct and invariant statement is that the classifying space functor is an equivalence of $(infty, 1)$-categories between grouplike $E_1$ spaces (this is a homotopically invariant version of "topological group") and pointed connected spaces (and every time I say "spaces" I mean "weak homotopy types"). I believe it's also true that every grouplike $E_1$ space can be modeled by a topological group.
Trying to strictify this equivalence to a statement about homomorphisms beween topological groups is hopeless in general. The maps corresponding to maps $BG to BH$ are maps $G to H$ that respect the group operation up to coherent homotopy, so there's no reason to expect the classifying space functor, as a functor on topological groups and taking values in homotopy categories, to be full.
Explicitly, take $G$ to be discrete and take $H = S^1$. Then $[BG, BH] cong H^2(G, mathbbZ)$, which by the universal coefficient theorem has two summands, namely
$$textExt^1(H_1(G, mathbbZ), mathbbZ)$$
and $textHom(H_2(G, mathbbZ), mathbbZ)$. Group homomorphisms $G to S^1$ only give you terms in the first summand, via the Ext long exact sequence coming from the short exact sequence
$$0 to mathbbZ to mathbbR to S^1 to 0$$
giving a long exact sequence with connecting map $textHom(H_1(G), S^1) to textExt^1(H_1(G, mathbbZ), mathbbZ)$.
The second summand $textHom(H_2(G, mathbbZ), mathbbZ)$ is related to the nontrivial structure implied by respecting group operations up to coherent homotopy. (This means that $f(g_1 g_2)$ isn't equal to $f(g_1) f(g_2)$, but that there's a homotopy relating them which itself is required to satisfy certain axioms. This homotopy is an embellished version of a $2$-cocycle.)
An earlier version of this answer claimed a counterexample for compact Lie groups (the above example with $G$ finite) that doesn't work; thanks to Omar Antolín-Camarena for pointing this out. I don't know enough about e.g. the homotopy theory of $BSU(2)$ to write down a counterexample for compact Lie groups. I might start by looking at $G = S^1, H = SU(2)$.
$endgroup$
The correct and invariant statement is that the classifying space functor is an equivalence of $(infty, 1)$-categories between grouplike $E_1$ spaces (this is a homotopically invariant version of "topological group") and pointed connected spaces (and every time I say "spaces" I mean "weak homotopy types"). I believe it's also true that every grouplike $E_1$ space can be modeled by a topological group.
Trying to strictify this equivalence to a statement about homomorphisms beween topological groups is hopeless in general. The maps corresponding to maps $BG to BH$ are maps $G to H$ that respect the group operation up to coherent homotopy, so there's no reason to expect the classifying space functor, as a functor on topological groups and taking values in homotopy categories, to be full.
Explicitly, take $G$ to be discrete and take $H = S^1$. Then $[BG, BH] cong H^2(G, mathbbZ)$, which by the universal coefficient theorem has two summands, namely
$$textExt^1(H_1(G, mathbbZ), mathbbZ)$$
and $textHom(H_2(G, mathbbZ), mathbbZ)$. Group homomorphisms $G to S^1$ only give you terms in the first summand, via the Ext long exact sequence coming from the short exact sequence
$$0 to mathbbZ to mathbbR to S^1 to 0$$
giving a long exact sequence with connecting map $textHom(H_1(G), S^1) to textExt^1(H_1(G, mathbbZ), mathbbZ)$.
The second summand $textHom(H_2(G, mathbbZ), mathbbZ)$ is related to the nontrivial structure implied by respecting group operations up to coherent homotopy. (This means that $f(g_1 g_2)$ isn't equal to $f(g_1) f(g_2)$, but that there's a homotopy relating them which itself is required to satisfy certain axioms. This homotopy is an embellished version of a $2$-cocycle.)
An earlier version of this answer claimed a counterexample for compact Lie groups (the above example with $G$ finite) that doesn't work; thanks to Omar Antolín-Camarena for pointing this out. I don't know enough about e.g. the homotopy theory of $BSU(2)$ to write down a counterexample for compact Lie groups. I might start by looking at $G = S^1, H = SU(2)$.
edited Apr 22 '16 at 2:09
answered Oct 1 '15 at 2:49
Qiaochu YuanQiaochu Yuan
281k32593938
281k32593938
1
$begingroup$
Homotopy clases of continuos group homomorphisms are the wrong thing to look at only if they are not what you have to look at. That sort of claim is only true in situations where one has nothing to do but deontics! :-)
$endgroup$
– Mariano Suárez-Álvarez
Oct 1 '15 at 2:58
$begingroup$
@MarianoSuárez-Alvarez: I'm not sure I agree. In any case, I learned something from this answer.
$endgroup$
– user98602
Oct 1 '15 at 3:32
1
$begingroup$
@Mike: in general, when $H = U(n)$ this issue is closely related to the difference between Borel-equivariant and genuinely equivariant K-theory (of a point); Borel $G$-equivariant K-theory of a point is the K-theory of $BG$, while genuinely $G$-equivariant K-theory of a point is the representation theory of $G$. These two differ but are related by the Atiyah-Segal completion theorem.
$endgroup$
– Qiaochu Yuan
Oct 1 '15 at 16:52
1
$begingroup$
Isn't the other summand in $H^2(G,mathbbZ)$ given by $mathrmHom(H_2(G,mathbbZ), mathbbZ)$, which vanishes because for a finite group $G$, $H_2(G, mathbbZ)$ is finite too?
$endgroup$
– Omar Antolín-Camarena
Apr 22 '16 at 1:33
1
$begingroup$
In Maps of BZ/p to BG, Dwyer and Wilkerson show that the map Hom(G, SU(2))/conjugation --> [BG, BSU(2)] (that's homotopy classes of maps, not of pointed maps, which is why the domain is conjugation classes of group homomorphisms) is not surjective if G is the symmetric group on 3 letters and also that it is not injective if G is cyclic of order 15.
$endgroup$
– Omar Antolín-Camarena
Apr 22 '16 at 17:46
|
show 4 more comments
1
$begingroup$
Homotopy clases of continuos group homomorphisms are the wrong thing to look at only if they are not what you have to look at. That sort of claim is only true in situations where one has nothing to do but deontics! :-)
$endgroup$
– Mariano Suárez-Álvarez
Oct 1 '15 at 2:58
$begingroup$
@MarianoSuárez-Alvarez: I'm not sure I agree. In any case, I learned something from this answer.
$endgroup$
– user98602
Oct 1 '15 at 3:32
1
$begingroup$
@Mike: in general, when $H = U(n)$ this issue is closely related to the difference between Borel-equivariant and genuinely equivariant K-theory (of a point); Borel $G$-equivariant K-theory of a point is the K-theory of $BG$, while genuinely $G$-equivariant K-theory of a point is the representation theory of $G$. These two differ but are related by the Atiyah-Segal completion theorem.
$endgroup$
– Qiaochu Yuan
Oct 1 '15 at 16:52
1
$begingroup$
Isn't the other summand in $H^2(G,mathbbZ)$ given by $mathrmHom(H_2(G,mathbbZ), mathbbZ)$, which vanishes because for a finite group $G$, $H_2(G, mathbbZ)$ is finite too?
$endgroup$
– Omar Antolín-Camarena
Apr 22 '16 at 1:33
1
$begingroup$
In Maps of BZ/p to BG, Dwyer and Wilkerson show that the map Hom(G, SU(2))/conjugation --> [BG, BSU(2)] (that's homotopy classes of maps, not of pointed maps, which is why the domain is conjugation classes of group homomorphisms) is not surjective if G is the symmetric group on 3 letters and also that it is not injective if G is cyclic of order 15.
$endgroup$
– Omar Antolín-Camarena
Apr 22 '16 at 17:46
1
1
$begingroup$
Homotopy clases of continuos group homomorphisms are the wrong thing to look at only if they are not what you have to look at. That sort of claim is only true in situations where one has nothing to do but deontics! :-)
$endgroup$
– Mariano Suárez-Álvarez
Oct 1 '15 at 2:58
$begingroup$
Homotopy clases of continuos group homomorphisms are the wrong thing to look at only if they are not what you have to look at. That sort of claim is only true in situations where one has nothing to do but deontics! :-)
$endgroup$
– Mariano Suárez-Álvarez
Oct 1 '15 at 2:58
$begingroup$
@MarianoSuárez-Alvarez: I'm not sure I agree. In any case, I learned something from this answer.
$endgroup$
– user98602
Oct 1 '15 at 3:32
$begingroup$
@MarianoSuárez-Alvarez: I'm not sure I agree. In any case, I learned something from this answer.
$endgroup$
– user98602
Oct 1 '15 at 3:32
1
1
$begingroup$
@Mike: in general, when $H = U(n)$ this issue is closely related to the difference between Borel-equivariant and genuinely equivariant K-theory (of a point); Borel $G$-equivariant K-theory of a point is the K-theory of $BG$, while genuinely $G$-equivariant K-theory of a point is the representation theory of $G$. These two differ but are related by the Atiyah-Segal completion theorem.
$endgroup$
– Qiaochu Yuan
Oct 1 '15 at 16:52
$begingroup$
@Mike: in general, when $H = U(n)$ this issue is closely related to the difference between Borel-equivariant and genuinely equivariant K-theory (of a point); Borel $G$-equivariant K-theory of a point is the K-theory of $BG$, while genuinely $G$-equivariant K-theory of a point is the representation theory of $G$. These two differ but are related by the Atiyah-Segal completion theorem.
$endgroup$
– Qiaochu Yuan
Oct 1 '15 at 16:52
1
1
$begingroup$
Isn't the other summand in $H^2(G,mathbbZ)$ given by $mathrmHom(H_2(G,mathbbZ), mathbbZ)$, which vanishes because for a finite group $G$, $H_2(G, mathbbZ)$ is finite too?
$endgroup$
– Omar Antolín-Camarena
Apr 22 '16 at 1:33
$begingroup$
Isn't the other summand in $H^2(G,mathbbZ)$ given by $mathrmHom(H_2(G,mathbbZ), mathbbZ)$, which vanishes because for a finite group $G$, $H_2(G, mathbbZ)$ is finite too?
$endgroup$
– Omar Antolín-Camarena
Apr 22 '16 at 1:33
1
1
$begingroup$
In Maps of BZ/p to BG, Dwyer and Wilkerson show that the map Hom(G, SU(2))/conjugation --> [BG, BSU(2)] (that's homotopy classes of maps, not of pointed maps, which is why the domain is conjugation classes of group homomorphisms) is not surjective if G is the symmetric group on 3 letters and also that it is not injective if G is cyclic of order 15.
$endgroup$
– Omar Antolín-Camarena
Apr 22 '16 at 17:46
$begingroup$
In Maps of BZ/p to BG, Dwyer and Wilkerson show that the map Hom(G, SU(2))/conjugation --> [BG, BSU(2)] (that's homotopy classes of maps, not of pointed maps, which is why the domain is conjugation classes of group homomorphisms) is not surjective if G is the symmetric group on 3 letters and also that it is not injective if G is cyclic of order 15.
$endgroup$
– Omar Antolín-Camarena
Apr 22 '16 at 17:46
|
show 4 more comments
$begingroup$
The functor $B : mathbfTopGrp to operatornameHo mathbfTop_*$ is not fully faithful. Indeed, it does not even preserve non-isomorphy: the multiplicative group $mathbbR^times$ and the discrete group $mathbbZ / 2 mathbbZ$ have the same classifying space (namely, $mathbbR P^infty$), so if the functor were fully faithful, $mathbbR^times$ and $mathbbZ / 2 mathbbZ$ would have to be isomorphic as topological groups – but they are not.
$endgroup$
$begingroup$
How annoyingly trivial. Is this fixable by changing the former category or expanding the latter? (Perhaps by restricting to compact groups?)
$endgroup$
– user98602
Sep 30 '15 at 19:03
$begingroup$
Well, at minimum you should replace $mathbfTopGrp$ with $operatornameHo mathbfTopGrp$. But even then I suspect the answer is no.
$endgroup$
– Zhen Lin
Sep 30 '15 at 19:04
$begingroup$
I agree that is a better replacement than restricting to compact groups (though I don't know what the morphisms in your homotopy category are - homotopies through homomorphisms?) Please let me know if you find a contradiction to this statement.
$endgroup$
– user98602
Sep 30 '15 at 19:10
$begingroup$
Homotopies through homomorphisms is probably the simplest option. We would need to have induced homotopies of the morphisms of bar constructions.
$endgroup$
– Zhen Lin
Sep 30 '15 at 19:40
add a comment |
$begingroup$
The functor $B : mathbfTopGrp to operatornameHo mathbfTop_*$ is not fully faithful. Indeed, it does not even preserve non-isomorphy: the multiplicative group $mathbbR^times$ and the discrete group $mathbbZ / 2 mathbbZ$ have the same classifying space (namely, $mathbbR P^infty$), so if the functor were fully faithful, $mathbbR^times$ and $mathbbZ / 2 mathbbZ$ would have to be isomorphic as topological groups – but they are not.
$endgroup$
$begingroup$
How annoyingly trivial. Is this fixable by changing the former category or expanding the latter? (Perhaps by restricting to compact groups?)
$endgroup$
– user98602
Sep 30 '15 at 19:03
$begingroup$
Well, at minimum you should replace $mathbfTopGrp$ with $operatornameHo mathbfTopGrp$. But even then I suspect the answer is no.
$endgroup$
– Zhen Lin
Sep 30 '15 at 19:04
$begingroup$
I agree that is a better replacement than restricting to compact groups (though I don't know what the morphisms in your homotopy category are - homotopies through homomorphisms?) Please let me know if you find a contradiction to this statement.
$endgroup$
– user98602
Sep 30 '15 at 19:10
$begingroup$
Homotopies through homomorphisms is probably the simplest option. We would need to have induced homotopies of the morphisms of bar constructions.
$endgroup$
– Zhen Lin
Sep 30 '15 at 19:40
add a comment |
$begingroup$
The functor $B : mathbfTopGrp to operatornameHo mathbfTop_*$ is not fully faithful. Indeed, it does not even preserve non-isomorphy: the multiplicative group $mathbbR^times$ and the discrete group $mathbbZ / 2 mathbbZ$ have the same classifying space (namely, $mathbbR P^infty$), so if the functor were fully faithful, $mathbbR^times$ and $mathbbZ / 2 mathbbZ$ would have to be isomorphic as topological groups – but they are not.
$endgroup$
The functor $B : mathbfTopGrp to operatornameHo mathbfTop_*$ is not fully faithful. Indeed, it does not even preserve non-isomorphy: the multiplicative group $mathbbR^times$ and the discrete group $mathbbZ / 2 mathbbZ$ have the same classifying space (namely, $mathbbR P^infty$), so if the functor were fully faithful, $mathbbR^times$ and $mathbbZ / 2 mathbbZ$ would have to be isomorphic as topological groups – but they are not.
answered Sep 30 '15 at 19:01
Zhen LinZhen Lin
60.8k4110227
60.8k4110227
$begingroup$
How annoyingly trivial. Is this fixable by changing the former category or expanding the latter? (Perhaps by restricting to compact groups?)
$endgroup$
– user98602
Sep 30 '15 at 19:03
$begingroup$
Well, at minimum you should replace $mathbfTopGrp$ with $operatornameHo mathbfTopGrp$. But even then I suspect the answer is no.
$endgroup$
– Zhen Lin
Sep 30 '15 at 19:04
$begingroup$
I agree that is a better replacement than restricting to compact groups (though I don't know what the morphisms in your homotopy category are - homotopies through homomorphisms?) Please let me know if you find a contradiction to this statement.
$endgroup$
– user98602
Sep 30 '15 at 19:10
$begingroup$
Homotopies through homomorphisms is probably the simplest option. We would need to have induced homotopies of the morphisms of bar constructions.
$endgroup$
– Zhen Lin
Sep 30 '15 at 19:40
add a comment |
$begingroup$
How annoyingly trivial. Is this fixable by changing the former category or expanding the latter? (Perhaps by restricting to compact groups?)
$endgroup$
– user98602
Sep 30 '15 at 19:03
$begingroup$
Well, at minimum you should replace $mathbfTopGrp$ with $operatornameHo mathbfTopGrp$. But even then I suspect the answer is no.
$endgroup$
– Zhen Lin
Sep 30 '15 at 19:04
$begingroup$
I agree that is a better replacement than restricting to compact groups (though I don't know what the morphisms in your homotopy category are - homotopies through homomorphisms?) Please let me know if you find a contradiction to this statement.
$endgroup$
– user98602
Sep 30 '15 at 19:10
$begingroup$
Homotopies through homomorphisms is probably the simplest option. We would need to have induced homotopies of the morphisms of bar constructions.
$endgroup$
– Zhen Lin
Sep 30 '15 at 19:40
$begingroup$
How annoyingly trivial. Is this fixable by changing the former category or expanding the latter? (Perhaps by restricting to compact groups?)
$endgroup$
– user98602
Sep 30 '15 at 19:03
$begingroup$
How annoyingly trivial. Is this fixable by changing the former category or expanding the latter? (Perhaps by restricting to compact groups?)
$endgroup$
– user98602
Sep 30 '15 at 19:03
$begingroup$
Well, at minimum you should replace $mathbfTopGrp$ with $operatornameHo mathbfTopGrp$. But even then I suspect the answer is no.
$endgroup$
– Zhen Lin
Sep 30 '15 at 19:04
$begingroup$
Well, at minimum you should replace $mathbfTopGrp$ with $operatornameHo mathbfTopGrp$. But even then I suspect the answer is no.
$endgroup$
– Zhen Lin
Sep 30 '15 at 19:04
$begingroup$
I agree that is a better replacement than restricting to compact groups (though I don't know what the morphisms in your homotopy category are - homotopies through homomorphisms?) Please let me know if you find a contradiction to this statement.
$endgroup$
– user98602
Sep 30 '15 at 19:10
$begingroup$
I agree that is a better replacement than restricting to compact groups (though I don't know what the morphisms in your homotopy category are - homotopies through homomorphisms?) Please let me know if you find a contradiction to this statement.
$endgroup$
– user98602
Sep 30 '15 at 19:10
$begingroup$
Homotopies through homomorphisms is probably the simplest option. We would need to have induced homotopies of the morphisms of bar constructions.
$endgroup$
– Zhen Lin
Sep 30 '15 at 19:40
$begingroup$
Homotopies through homomorphisms is probably the simplest option. We would need to have induced homotopies of the morphisms of bar constructions.
$endgroup$
– Zhen Lin
Sep 30 '15 at 19:40
add a comment |
$begingroup$
I know this is an old+answered question, but I don't think the following result has been explicitly brought up:
Let $G$ be a Lie group, $gin G$, and let $c_gcolon Gto G$ be the automorphism given by $c_g(h) = ghg^-1$. Then the induced map $B c_gcolon BG to BG$ is homotopic to the identity map.
(see for example Ebert's Notes on index theory, lemma 9.2.2 on page 143)
Then any $(G, g)$ such that $c_g$ is not homotopic to the identity map (for example $G$ discrete and $g$ not in the centre) is a counter-example.
Based on Omar Antolín-Camarena's comment on Qiaochu Yuan's answer, it seems like quotienting out the inner automorphisms would not be enough to make it fully-faithful in general either.
$endgroup$
add a comment |
$begingroup$
I know this is an old+answered question, but I don't think the following result has been explicitly brought up:
Let $G$ be a Lie group, $gin G$, and let $c_gcolon Gto G$ be the automorphism given by $c_g(h) = ghg^-1$. Then the induced map $B c_gcolon BG to BG$ is homotopic to the identity map.
(see for example Ebert's Notes on index theory, lemma 9.2.2 on page 143)
Then any $(G, g)$ such that $c_g$ is not homotopic to the identity map (for example $G$ discrete and $g$ not in the centre) is a counter-example.
Based on Omar Antolín-Camarena's comment on Qiaochu Yuan's answer, it seems like quotienting out the inner automorphisms would not be enough to make it fully-faithful in general either.
$endgroup$
add a comment |
$begingroup$
I know this is an old+answered question, but I don't think the following result has been explicitly brought up:
Let $G$ be a Lie group, $gin G$, and let $c_gcolon Gto G$ be the automorphism given by $c_g(h) = ghg^-1$. Then the induced map $B c_gcolon BG to BG$ is homotopic to the identity map.
(see for example Ebert's Notes on index theory, lemma 9.2.2 on page 143)
Then any $(G, g)$ such that $c_g$ is not homotopic to the identity map (for example $G$ discrete and $g$ not in the centre) is a counter-example.
Based on Omar Antolín-Camarena's comment on Qiaochu Yuan's answer, it seems like quotienting out the inner automorphisms would not be enough to make it fully-faithful in general either.
$endgroup$
I know this is an old+answered question, but I don't think the following result has been explicitly brought up:
Let $G$ be a Lie group, $gin G$, and let $c_gcolon Gto G$ be the automorphism given by $c_g(h) = ghg^-1$. Then the induced map $B c_gcolon BG to BG$ is homotopic to the identity map.
(see for example Ebert's Notes on index theory, lemma 9.2.2 on page 143)
Then any $(G, g)$ such that $c_g$ is not homotopic to the identity map (for example $G$ discrete and $g$ not in the centre) is a counter-example.
Based on Omar Antolín-Camarena's comment on Qiaochu Yuan's answer, it seems like quotienting out the inner automorphisms would not be enough to make it fully-faithful in general either.
answered Mar 15 at 15:14
WilliamWilliam
2,8101224
2,8101224
add a comment |
add a comment |
Thanks for contributing an answer to Mathematics Stack Exchange!
- Please be sure to answer the question. Provide details and share your research!
But avoid …
- Asking for help, clarification, or responding to other answers.
- Making statements based on opinion; back them up with references or personal experience.
Use MathJax to format equations. MathJax reference.
To learn more, see our tips on writing great answers.
Sign up or log in
StackExchange.ready(function ()
StackExchange.helpers.onClickDraftSave('#login-link');
);
Sign up using Google
Sign up using Facebook
Sign up using Email and Password
Post as a guest
Required, but never shown
StackExchange.ready(
function ()
StackExchange.openid.initPostLogin('.new-post-login', 'https%3a%2f%2fmath.stackexchange.com%2fquestions%2f1458470%2fis-the-classifying-space-a-fully-faithful-functor%23new-answer', 'question_page');
);
Post as a guest
Required, but never shown
Sign up or log in
StackExchange.ready(function ()
StackExchange.helpers.onClickDraftSave('#login-link');
);
Sign up using Google
Sign up using Facebook
Sign up using Email and Password
Post as a guest
Required, but never shown
Sign up or log in
StackExchange.ready(function ()
StackExchange.helpers.onClickDraftSave('#login-link');
);
Sign up using Google
Sign up using Facebook
Sign up using Email and Password
Post as a guest
Required, but never shown
Sign up or log in
StackExchange.ready(function ()
StackExchange.helpers.onClickDraftSave('#login-link');
);
Sign up using Google
Sign up using Facebook
Sign up using Email and Password
Sign up using Google
Sign up using Facebook
Sign up using Email and Password
Post as a guest
Required, but never shown
Required, but never shown
Required, but never shown
Required, but never shown
Required, but never shown
Required, but never shown
Required, but never shown
Required, but never shown
Required, but never shown
2
$begingroup$
It's not even true for discrete groups unless you go to the pointed homotopy category.
$endgroup$
– Zhen Lin
Sep 30 '15 at 18:52
$begingroup$
@ZhenLin Fair enough, that's what I meant. Let me edit.
$endgroup$
– user98602
Sep 30 '15 at 18:53