Why does this trick to derive the formula for $[A^n,B]$ in terms of repeated commutators work so well? Announcing the arrival of Valued Associate #679: Cesar Manara Planned maintenance scheduled April 17/18, 2019 at 00:00UTC (8:00pm US/Eastern)Why does this method for solving matrix equations work?Why does this algorithm work?why does the blockwise inversion formula work?How does one derive this rotation quaternion formula?Why does this expression for the minimum eigenvalue work?The expression “commute to something”Why does this “miracle method” for matrix inversion work?Why doesn't the general formula $(A - lambda I) eta = v$ for Eigenvectors of repeated eigenvalues work?Why does this method for finding basis work?Why does this least-squares approach to proving the Sherman–Morrison Formula work?
What exactly is a "Meth" in Altered Carbon?
English words in a non-english sci-fi novel
Sci-Fi book where patients in a coma ward all live in a subconscious world linked together
What is the meaning of the new sigil in Game of Thrones Season 8 intro?
Why did the Falcon Heavy center core fall off the ASDS OCISLY barge?
Is the Standard Deduction better than Itemized when both are the same amount?
Why do we bend a book to keep it straight?
Why aren't air breathing engines used as small first stages
How do pianists reach extremely loud dynamics?
Can a USB port passively 'listen only'?
List *all* the tuples!
How to react to hostile behavior from a senior developer?
2001: A Space Odyssey's use of the song "Daisy Bell" (Bicycle Built for Two); life imitates art or vice-versa?
Should I discuss the type of campaign with my players?
Error "illegal generic type for instanceof" when using local classes
Output the ŋarâþ crîþ alphabet song without using (m)any letters
What is Arya's weapon design?
Why did the IBM 650 use bi-quinary?
List of Python versions
porting install scripts : can rpm replace apt?
Okay to merge included columns on otherwise identical indexes?
How does the particle を relate to the verb 行く in the structure「A を + B に行く」?
Can a non-EU citizen traveling with me come with me through the EU passport line?
What causes the vertical darker bands in my photo?
Why does this trick to derive the formula for $[A^n,B]$ in terms of repeated commutators work so well?
Announcing the arrival of Valued Associate #679: Cesar Manara
Planned maintenance scheduled April 17/18, 2019 at 00:00UTC (8:00pm US/Eastern)Why does this method for solving matrix equations work?Why does this algorithm work?why does the blockwise inversion formula work?How does one derive this rotation quaternion formula?Why does this expression for the minimum eigenvalue work?The expression “commute to something”Why does this “miracle method” for matrix inversion work?Why doesn't the general formula $(A - lambda I) eta = v$ for Eigenvectors of repeated eigenvalues work?Why does this method for finding basis work?Why does this least-squares approach to proving the Sherman–Morrison Formula work?
$begingroup$
It is a known result that, given generically noncommuting operators $A,B$, we have
$$ A^n B=sum_k=0^n binomnk operatornamead^k(A)(B) A^n-k,tag A $$
where $operatornamead^k(A)(B)equiv[underbraceA,[A,[dots,[A_k,B]dots]] $.
This can be proved for example via induction with not too much work.
However, while trying to get a better understanding of this formula, I realised that there is a much easier way to derive it, at least on a formal, intuitive level.
The trick
Let $hatmathcal S$ and $hatmathcal C$ (standing for "shift" and "commute", respectively) denote operators that act on expressions of the form $A^k D^j A^ell$ (denoting for simplicity $D^jequivoperatornamead^j(A)(B)$) as follows:
beginalign
hatmathcal S (A^k D^j A^ell)
&= A^k-1 D^j A^ell+1, \
hatmathcal C (A^k D^j A^ell)
&= A^k-1 D^j+1 A^ell.
endalign
In other words, $hatmathcal S$ "moves" the central $D$ block on the left, while $hatmathcal C$ makes it "eat" the neighboring $A$ factor.
It is not hard to see that $hatmathcal S+hatmathcal C=mathbb 1$, which is but another way to state the identity
$$A[A,B]=[A,B]A+[A,[A,B]].$$
Moreover, crucially, $hatmathcal S$ and $hatmathcal C$ commute.
Because of this, I can write
$$A^n B=(hatmathcal S+hatmathcal C)^n (A^n B)=sum_k=0^nbinomnk hatmathcal S^n-k hatmathcal C^k(A^n B),$$
which immediately gives me (A) without any need for recursion or other tricks.
The question
Now, this is all fine and dandy, but it leaves me wondering as to why does this kind of thing work?
It looks like I am somehow bypassing the nuisance of having to deal with non-commuting operations by switching to a space of "superoperators", in which the same operation can be expressed in terms of commuting "superoperators".
I am not even sure how one could go in formalising this "superoperators" $hatmathcal S,hatmathcal C$, as they seem to be objects acting on "strings of operators" more than on the elements of the operator algebra themselves.
Is there a way to formalise this way of handling the expressions? Is this a well-known method in this context (I had never seen it but I am not well-versed in this kinds of manipulations)?
linear-algebra matrices operator-algebras noncommutative-algebra
$endgroup$
add a comment |
$begingroup$
It is a known result that, given generically noncommuting operators $A,B$, we have
$$ A^n B=sum_k=0^n binomnk operatornamead^k(A)(B) A^n-k,tag A $$
where $operatornamead^k(A)(B)equiv[underbraceA,[A,[dots,[A_k,B]dots]] $.
This can be proved for example via induction with not too much work.
However, while trying to get a better understanding of this formula, I realised that there is a much easier way to derive it, at least on a formal, intuitive level.
The trick
Let $hatmathcal S$ and $hatmathcal C$ (standing for "shift" and "commute", respectively) denote operators that act on expressions of the form $A^k D^j A^ell$ (denoting for simplicity $D^jequivoperatornamead^j(A)(B)$) as follows:
beginalign
hatmathcal S (A^k D^j A^ell)
&= A^k-1 D^j A^ell+1, \
hatmathcal C (A^k D^j A^ell)
&= A^k-1 D^j+1 A^ell.
endalign
In other words, $hatmathcal S$ "moves" the central $D$ block on the left, while $hatmathcal C$ makes it "eat" the neighboring $A$ factor.
It is not hard to see that $hatmathcal S+hatmathcal C=mathbb 1$, which is but another way to state the identity
$$A[A,B]=[A,B]A+[A,[A,B]].$$
Moreover, crucially, $hatmathcal S$ and $hatmathcal C$ commute.
Because of this, I can write
$$A^n B=(hatmathcal S+hatmathcal C)^n (A^n B)=sum_k=0^nbinomnk hatmathcal S^n-k hatmathcal C^k(A^n B),$$
which immediately gives me (A) without any need for recursion or other tricks.
The question
Now, this is all fine and dandy, but it leaves me wondering as to why does this kind of thing work?
It looks like I am somehow bypassing the nuisance of having to deal with non-commuting operations by switching to a space of "superoperators", in which the same operation can be expressed in terms of commuting "superoperators".
I am not even sure how one could go in formalising this "superoperators" $hatmathcal S,hatmathcal C$, as they seem to be objects acting on "strings of operators" more than on the elements of the operator algebra themselves.
Is there a way to formalise this way of handling the expressions? Is this a well-known method in this context (I had never seen it but I am not well-versed in this kinds of manipulations)?
linear-algebra matrices operator-algebras noncommutative-algebra
$endgroup$
$begingroup$
First note that your trick does use recursion, in the proof of the binomial theorem, so you didn't actually get rid of the recursion, only pushed it further away from sight. Secondly, I don't know how general you're trying to see things, but your trick can probably be formalized using some free noncommutative algebra on $A,B,D$
$endgroup$
– Max
Mar 26 at 19:06
$begingroup$
@Max yes ok, I didn't mean to say that I wanted a way to completely get rid of recursion for its own sake, the point was more about having a more direct way to get to the result (but also, do you really need recursion to prove the binomial theorem? I would have said that a combinatorial argument is more than sufficient for that. Then again, recursion might be used for the combinatorial formulas I guess). I am not familiar with "free algebras", but from a quick read on wikipedia it seems to be kind of a way to formalize acting on strings of "formal objects", so it seems on point.
$endgroup$
– glS
Mar 26 at 19:12
add a comment |
$begingroup$
It is a known result that, given generically noncommuting operators $A,B$, we have
$$ A^n B=sum_k=0^n binomnk operatornamead^k(A)(B) A^n-k,tag A $$
where $operatornamead^k(A)(B)equiv[underbraceA,[A,[dots,[A_k,B]dots]] $.
This can be proved for example via induction with not too much work.
However, while trying to get a better understanding of this formula, I realised that there is a much easier way to derive it, at least on a formal, intuitive level.
The trick
Let $hatmathcal S$ and $hatmathcal C$ (standing for "shift" and "commute", respectively) denote operators that act on expressions of the form $A^k D^j A^ell$ (denoting for simplicity $D^jequivoperatornamead^j(A)(B)$) as follows:
beginalign
hatmathcal S (A^k D^j A^ell)
&= A^k-1 D^j A^ell+1, \
hatmathcal C (A^k D^j A^ell)
&= A^k-1 D^j+1 A^ell.
endalign
In other words, $hatmathcal S$ "moves" the central $D$ block on the left, while $hatmathcal C$ makes it "eat" the neighboring $A$ factor.
It is not hard to see that $hatmathcal S+hatmathcal C=mathbb 1$, which is but another way to state the identity
$$A[A,B]=[A,B]A+[A,[A,B]].$$
Moreover, crucially, $hatmathcal S$ and $hatmathcal C$ commute.
Because of this, I can write
$$A^n B=(hatmathcal S+hatmathcal C)^n (A^n B)=sum_k=0^nbinomnk hatmathcal S^n-k hatmathcal C^k(A^n B),$$
which immediately gives me (A) without any need for recursion or other tricks.
The question
Now, this is all fine and dandy, but it leaves me wondering as to why does this kind of thing work?
It looks like I am somehow bypassing the nuisance of having to deal with non-commuting operations by switching to a space of "superoperators", in which the same operation can be expressed in terms of commuting "superoperators".
I am not even sure how one could go in formalising this "superoperators" $hatmathcal S,hatmathcal C$, as they seem to be objects acting on "strings of operators" more than on the elements of the operator algebra themselves.
Is there a way to formalise this way of handling the expressions? Is this a well-known method in this context (I had never seen it but I am not well-versed in this kinds of manipulations)?
linear-algebra matrices operator-algebras noncommutative-algebra
$endgroup$
It is a known result that, given generically noncommuting operators $A,B$, we have
$$ A^n B=sum_k=0^n binomnk operatornamead^k(A)(B) A^n-k,tag A $$
where $operatornamead^k(A)(B)equiv[underbraceA,[A,[dots,[A_k,B]dots]] $.
This can be proved for example via induction with not too much work.
However, while trying to get a better understanding of this formula, I realised that there is a much easier way to derive it, at least on a formal, intuitive level.
The trick
Let $hatmathcal S$ and $hatmathcal C$ (standing for "shift" and "commute", respectively) denote operators that act on expressions of the form $A^k D^j A^ell$ (denoting for simplicity $D^jequivoperatornamead^j(A)(B)$) as follows:
beginalign
hatmathcal S (A^k D^j A^ell)
&= A^k-1 D^j A^ell+1, \
hatmathcal C (A^k D^j A^ell)
&= A^k-1 D^j+1 A^ell.
endalign
In other words, $hatmathcal S$ "moves" the central $D$ block on the left, while $hatmathcal C$ makes it "eat" the neighboring $A$ factor.
It is not hard to see that $hatmathcal S+hatmathcal C=mathbb 1$, which is but another way to state the identity
$$A[A,B]=[A,B]A+[A,[A,B]].$$
Moreover, crucially, $hatmathcal S$ and $hatmathcal C$ commute.
Because of this, I can write
$$A^n B=(hatmathcal S+hatmathcal C)^n (A^n B)=sum_k=0^nbinomnk hatmathcal S^n-k hatmathcal C^k(A^n B),$$
which immediately gives me (A) without any need for recursion or other tricks.
The question
Now, this is all fine and dandy, but it leaves me wondering as to why does this kind of thing work?
It looks like I am somehow bypassing the nuisance of having to deal with non-commuting operations by switching to a space of "superoperators", in which the same operation can be expressed in terms of commuting "superoperators".
I am not even sure how one could go in formalising this "superoperators" $hatmathcal S,hatmathcal C$, as they seem to be objects acting on "strings of operators" more than on the elements of the operator algebra themselves.
Is there a way to formalise this way of handling the expressions? Is this a well-known method in this context (I had never seen it but I am not well-versed in this kinds of manipulations)?
linear-algebra matrices operator-algebras noncommutative-algebra
linear-algebra matrices operator-algebras noncommutative-algebra
edited Mar 26 at 18:57
glS
asked Mar 26 at 18:26
glSglS
790521
790521
$begingroup$
First note that your trick does use recursion, in the proof of the binomial theorem, so you didn't actually get rid of the recursion, only pushed it further away from sight. Secondly, I don't know how general you're trying to see things, but your trick can probably be formalized using some free noncommutative algebra on $A,B,D$
$endgroup$
– Max
Mar 26 at 19:06
$begingroup$
@Max yes ok, I didn't mean to say that I wanted a way to completely get rid of recursion for its own sake, the point was more about having a more direct way to get to the result (but also, do you really need recursion to prove the binomial theorem? I would have said that a combinatorial argument is more than sufficient for that. Then again, recursion might be used for the combinatorial formulas I guess). I am not familiar with "free algebras", but from a quick read on wikipedia it seems to be kind of a way to formalize acting on strings of "formal objects", so it seems on point.
$endgroup$
– glS
Mar 26 at 19:12
add a comment |
$begingroup$
First note that your trick does use recursion, in the proof of the binomial theorem, so you didn't actually get rid of the recursion, only pushed it further away from sight. Secondly, I don't know how general you're trying to see things, but your trick can probably be formalized using some free noncommutative algebra on $A,B,D$
$endgroup$
– Max
Mar 26 at 19:06
$begingroup$
@Max yes ok, I didn't mean to say that I wanted a way to completely get rid of recursion for its own sake, the point was more about having a more direct way to get to the result (but also, do you really need recursion to prove the binomial theorem? I would have said that a combinatorial argument is more than sufficient for that. Then again, recursion might be used for the combinatorial formulas I guess). I am not familiar with "free algebras", but from a quick read on wikipedia it seems to be kind of a way to formalize acting on strings of "formal objects", so it seems on point.
$endgroup$
– glS
Mar 26 at 19:12
$begingroup$
First note that your trick does use recursion, in the proof of the binomial theorem, so you didn't actually get rid of the recursion, only pushed it further away from sight. Secondly, I don't know how general you're trying to see things, but your trick can probably be formalized using some free noncommutative algebra on $A,B,D$
$endgroup$
– Max
Mar 26 at 19:06
$begingroup$
First note that your trick does use recursion, in the proof of the binomial theorem, so you didn't actually get rid of the recursion, only pushed it further away from sight. Secondly, I don't know how general you're trying to see things, but your trick can probably be formalized using some free noncommutative algebra on $A,B,D$
$endgroup$
– Max
Mar 26 at 19:06
$begingroup$
@Max yes ok, I didn't mean to say that I wanted a way to completely get rid of recursion for its own sake, the point was more about having a more direct way to get to the result (but also, do you really need recursion to prove the binomial theorem? I would have said that a combinatorial argument is more than sufficient for that. Then again, recursion might be used for the combinatorial formulas I guess). I am not familiar with "free algebras", but from a quick read on wikipedia it seems to be kind of a way to formalize acting on strings of "formal objects", so it seems on point.
$endgroup$
– glS
Mar 26 at 19:12
$begingroup$
@Max yes ok, I didn't mean to say that I wanted a way to completely get rid of recursion for its own sake, the point was more about having a more direct way to get to the result (but also, do you really need recursion to prove the binomial theorem? I would have said that a combinatorial argument is more than sufficient for that. Then again, recursion might be used for the combinatorial formulas I guess). I am not familiar with "free algebras", but from a quick read on wikipedia it seems to be kind of a way to formalize acting on strings of "formal objects", so it seems on point.
$endgroup$
– glS
Mar 26 at 19:12
add a comment |
1 Answer
1
active
oldest
votes
$begingroup$
To fix some notation, suppose that the operators $A$ and $B$ belong to a vector space $V$, and that we are working with strings of $m$ operators. (For example, in $A^k D^j A^l$ we have $m = k + j + l$). Rather than writing a product $A^k B^j$ as an element of $V$, we can instead write
$$ A^otimes k otimes B^otimes j = underbraceA otimes cdots otimes A_k otimes underbraceB otimes cdots otimes B_j in V^otimes m$$
an element of the $m$th tensor power of $V$. We have a linear multiplication map $mu: V^otimes m to V$, which is just composition of operators, so for example $mu(A^otimes k otimes B^otimes j) = A^k B^j$. So the idea will be to define $hatmathcalS$ and $hatmathcalC$ as linear operators $V^otimes m to V^otimes m$, check that they commute and add to give the identity, and then finally apply them to a particular tensor $A^otimes n otimes B$, which will give an identity much like the one you're after. Applying the multiplication $mu$ will then give the exact identity.
Defining the operators is not too hard. We can take $hatmathcalS, hatmathcalC : V^otimes m to V^otimes m$ to be defined by the formulas
$$ beginaligned
hatmathcalS(v_1 otimes v_2 otimes cdots otimes v_m) &= v_2 otimes cdots otimes v_m otimes v_1 \
hatmathcalC(v_1 otimes v_2 otimes cdots otimes v_m) &= v_1 otimes v_2 otimes cdots otimes v_m - v_2 otimes cdots otimes v_m otimes v_1 endaligned$$
We then check that these formulas do the right thing, for example we need to make sure that $mu(hatmathcalC^k (A^otimes m-1 otimes B)) = A^m-k-1 D^k$ and so on.
With those definitions, it is easy to see that $hatmathcalC = mathbb1 - hatmathcalS$, and so they also commute, since $hatmathcalChatmathcalS = hatmathcalShatmathcalC = hatmathcalS - hatmathcalS^2$. So we can write
$$ A^otimes n otimes B = (hatmathcalS + hatmathcalC)^n (A^otimes n otimes B) = sum_k=0^n binomnk (hatmathcalS^n-k hatmathcalC^k) (A^otimes n otimes B)$$
and finally applying $mu$ on both sides gives the formula you are after.
$endgroup$
$begingroup$
very nice, I didn't realize you could so simply define the operators via "rotations" of the character strings. Still I wonder, is there any kind of generality in this sort of approach to derive formulas? For example, I realised that one can also easily derive the explicit formula for the repeated commutator $operatornamead^k(A)B$ as $operatornamead^k(A)B=(A_L-A_R)^k B$, where $A_L, A_R$ denote "multiplication by A on the left/right". This trick can be similarly understood as operators in the tensor algebra of $V$. I wonder if something similar could work to derive, say, the BCH formula
$endgroup$
– glS
Mar 27 at 11:38
1
$begingroup$
I guess something swept under the rug in this answer was that you could actually define those operators on the tensor power. (It wasn’t obvious to me that I could make up a $hatmathcalC$ which lifted yours). Also, the fact that everything was an operator on the same tensor power. If you have to move between different tensor powers, things become a little more complicated and there is more to check. But as you say, using the tensor algebra would be the right replacement.
$endgroup$
– Joppy
Mar 27 at 11:42
add a comment |
Your Answer
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%2f3163565%2fwhy-does-this-trick-to-derive-the-formula-for-an-b-in-terms-of-repeated-com%23new-answer', 'question_page');
);
Post as a guest
Required, but never shown
1 Answer
1
active
oldest
votes
1 Answer
1
active
oldest
votes
active
oldest
votes
active
oldest
votes
$begingroup$
To fix some notation, suppose that the operators $A$ and $B$ belong to a vector space $V$, and that we are working with strings of $m$ operators. (For example, in $A^k D^j A^l$ we have $m = k + j + l$). Rather than writing a product $A^k B^j$ as an element of $V$, we can instead write
$$ A^otimes k otimes B^otimes j = underbraceA otimes cdots otimes A_k otimes underbraceB otimes cdots otimes B_j in V^otimes m$$
an element of the $m$th tensor power of $V$. We have a linear multiplication map $mu: V^otimes m to V$, which is just composition of operators, so for example $mu(A^otimes k otimes B^otimes j) = A^k B^j$. So the idea will be to define $hatmathcalS$ and $hatmathcalC$ as linear operators $V^otimes m to V^otimes m$, check that they commute and add to give the identity, and then finally apply them to a particular tensor $A^otimes n otimes B$, which will give an identity much like the one you're after. Applying the multiplication $mu$ will then give the exact identity.
Defining the operators is not too hard. We can take $hatmathcalS, hatmathcalC : V^otimes m to V^otimes m$ to be defined by the formulas
$$ beginaligned
hatmathcalS(v_1 otimes v_2 otimes cdots otimes v_m) &= v_2 otimes cdots otimes v_m otimes v_1 \
hatmathcalC(v_1 otimes v_2 otimes cdots otimes v_m) &= v_1 otimes v_2 otimes cdots otimes v_m - v_2 otimes cdots otimes v_m otimes v_1 endaligned$$
We then check that these formulas do the right thing, for example we need to make sure that $mu(hatmathcalC^k (A^otimes m-1 otimes B)) = A^m-k-1 D^k$ and so on.
With those definitions, it is easy to see that $hatmathcalC = mathbb1 - hatmathcalS$, and so they also commute, since $hatmathcalChatmathcalS = hatmathcalShatmathcalC = hatmathcalS - hatmathcalS^2$. So we can write
$$ A^otimes n otimes B = (hatmathcalS + hatmathcalC)^n (A^otimes n otimes B) = sum_k=0^n binomnk (hatmathcalS^n-k hatmathcalC^k) (A^otimes n otimes B)$$
and finally applying $mu$ on both sides gives the formula you are after.
$endgroup$
$begingroup$
very nice, I didn't realize you could so simply define the operators via "rotations" of the character strings. Still I wonder, is there any kind of generality in this sort of approach to derive formulas? For example, I realised that one can also easily derive the explicit formula for the repeated commutator $operatornamead^k(A)B$ as $operatornamead^k(A)B=(A_L-A_R)^k B$, where $A_L, A_R$ denote "multiplication by A on the left/right". This trick can be similarly understood as operators in the tensor algebra of $V$. I wonder if something similar could work to derive, say, the BCH formula
$endgroup$
– glS
Mar 27 at 11:38
1
$begingroup$
I guess something swept under the rug in this answer was that you could actually define those operators on the tensor power. (It wasn’t obvious to me that I could make up a $hatmathcalC$ which lifted yours). Also, the fact that everything was an operator on the same tensor power. If you have to move between different tensor powers, things become a little more complicated and there is more to check. But as you say, using the tensor algebra would be the right replacement.
$endgroup$
– Joppy
Mar 27 at 11:42
add a comment |
$begingroup$
To fix some notation, suppose that the operators $A$ and $B$ belong to a vector space $V$, and that we are working with strings of $m$ operators. (For example, in $A^k D^j A^l$ we have $m = k + j + l$). Rather than writing a product $A^k B^j$ as an element of $V$, we can instead write
$$ A^otimes k otimes B^otimes j = underbraceA otimes cdots otimes A_k otimes underbraceB otimes cdots otimes B_j in V^otimes m$$
an element of the $m$th tensor power of $V$. We have a linear multiplication map $mu: V^otimes m to V$, which is just composition of operators, so for example $mu(A^otimes k otimes B^otimes j) = A^k B^j$. So the idea will be to define $hatmathcalS$ and $hatmathcalC$ as linear operators $V^otimes m to V^otimes m$, check that they commute and add to give the identity, and then finally apply them to a particular tensor $A^otimes n otimes B$, which will give an identity much like the one you're after. Applying the multiplication $mu$ will then give the exact identity.
Defining the operators is not too hard. We can take $hatmathcalS, hatmathcalC : V^otimes m to V^otimes m$ to be defined by the formulas
$$ beginaligned
hatmathcalS(v_1 otimes v_2 otimes cdots otimes v_m) &= v_2 otimes cdots otimes v_m otimes v_1 \
hatmathcalC(v_1 otimes v_2 otimes cdots otimes v_m) &= v_1 otimes v_2 otimes cdots otimes v_m - v_2 otimes cdots otimes v_m otimes v_1 endaligned$$
We then check that these formulas do the right thing, for example we need to make sure that $mu(hatmathcalC^k (A^otimes m-1 otimes B)) = A^m-k-1 D^k$ and so on.
With those definitions, it is easy to see that $hatmathcalC = mathbb1 - hatmathcalS$, and so they also commute, since $hatmathcalChatmathcalS = hatmathcalShatmathcalC = hatmathcalS - hatmathcalS^2$. So we can write
$$ A^otimes n otimes B = (hatmathcalS + hatmathcalC)^n (A^otimes n otimes B) = sum_k=0^n binomnk (hatmathcalS^n-k hatmathcalC^k) (A^otimes n otimes B)$$
and finally applying $mu$ on both sides gives the formula you are after.
$endgroup$
$begingroup$
very nice, I didn't realize you could so simply define the operators via "rotations" of the character strings. Still I wonder, is there any kind of generality in this sort of approach to derive formulas? For example, I realised that one can also easily derive the explicit formula for the repeated commutator $operatornamead^k(A)B$ as $operatornamead^k(A)B=(A_L-A_R)^k B$, where $A_L, A_R$ denote "multiplication by A on the left/right". This trick can be similarly understood as operators in the tensor algebra of $V$. I wonder if something similar could work to derive, say, the BCH formula
$endgroup$
– glS
Mar 27 at 11:38
1
$begingroup$
I guess something swept under the rug in this answer was that you could actually define those operators on the tensor power. (It wasn’t obvious to me that I could make up a $hatmathcalC$ which lifted yours). Also, the fact that everything was an operator on the same tensor power. If you have to move between different tensor powers, things become a little more complicated and there is more to check. But as you say, using the tensor algebra would be the right replacement.
$endgroup$
– Joppy
Mar 27 at 11:42
add a comment |
$begingroup$
To fix some notation, suppose that the operators $A$ and $B$ belong to a vector space $V$, and that we are working with strings of $m$ operators. (For example, in $A^k D^j A^l$ we have $m = k + j + l$). Rather than writing a product $A^k B^j$ as an element of $V$, we can instead write
$$ A^otimes k otimes B^otimes j = underbraceA otimes cdots otimes A_k otimes underbraceB otimes cdots otimes B_j in V^otimes m$$
an element of the $m$th tensor power of $V$. We have a linear multiplication map $mu: V^otimes m to V$, which is just composition of operators, so for example $mu(A^otimes k otimes B^otimes j) = A^k B^j$. So the idea will be to define $hatmathcalS$ and $hatmathcalC$ as linear operators $V^otimes m to V^otimes m$, check that they commute and add to give the identity, and then finally apply them to a particular tensor $A^otimes n otimes B$, which will give an identity much like the one you're after. Applying the multiplication $mu$ will then give the exact identity.
Defining the operators is not too hard. We can take $hatmathcalS, hatmathcalC : V^otimes m to V^otimes m$ to be defined by the formulas
$$ beginaligned
hatmathcalS(v_1 otimes v_2 otimes cdots otimes v_m) &= v_2 otimes cdots otimes v_m otimes v_1 \
hatmathcalC(v_1 otimes v_2 otimes cdots otimes v_m) &= v_1 otimes v_2 otimes cdots otimes v_m - v_2 otimes cdots otimes v_m otimes v_1 endaligned$$
We then check that these formulas do the right thing, for example we need to make sure that $mu(hatmathcalC^k (A^otimes m-1 otimes B)) = A^m-k-1 D^k$ and so on.
With those definitions, it is easy to see that $hatmathcalC = mathbb1 - hatmathcalS$, and so they also commute, since $hatmathcalChatmathcalS = hatmathcalShatmathcalC = hatmathcalS - hatmathcalS^2$. So we can write
$$ A^otimes n otimes B = (hatmathcalS + hatmathcalC)^n (A^otimes n otimes B) = sum_k=0^n binomnk (hatmathcalS^n-k hatmathcalC^k) (A^otimes n otimes B)$$
and finally applying $mu$ on both sides gives the formula you are after.
$endgroup$
To fix some notation, suppose that the operators $A$ and $B$ belong to a vector space $V$, and that we are working with strings of $m$ operators. (For example, in $A^k D^j A^l$ we have $m = k + j + l$). Rather than writing a product $A^k B^j$ as an element of $V$, we can instead write
$$ A^otimes k otimes B^otimes j = underbraceA otimes cdots otimes A_k otimes underbraceB otimes cdots otimes B_j in V^otimes m$$
an element of the $m$th tensor power of $V$. We have a linear multiplication map $mu: V^otimes m to V$, which is just composition of operators, so for example $mu(A^otimes k otimes B^otimes j) = A^k B^j$. So the idea will be to define $hatmathcalS$ and $hatmathcalC$ as linear operators $V^otimes m to V^otimes m$, check that they commute and add to give the identity, and then finally apply them to a particular tensor $A^otimes n otimes B$, which will give an identity much like the one you're after. Applying the multiplication $mu$ will then give the exact identity.
Defining the operators is not too hard. We can take $hatmathcalS, hatmathcalC : V^otimes m to V^otimes m$ to be defined by the formulas
$$ beginaligned
hatmathcalS(v_1 otimes v_2 otimes cdots otimes v_m) &= v_2 otimes cdots otimes v_m otimes v_1 \
hatmathcalC(v_1 otimes v_2 otimes cdots otimes v_m) &= v_1 otimes v_2 otimes cdots otimes v_m - v_2 otimes cdots otimes v_m otimes v_1 endaligned$$
We then check that these formulas do the right thing, for example we need to make sure that $mu(hatmathcalC^k (A^otimes m-1 otimes B)) = A^m-k-1 D^k$ and so on.
With those definitions, it is easy to see that $hatmathcalC = mathbb1 - hatmathcalS$, and so they also commute, since $hatmathcalChatmathcalS = hatmathcalShatmathcalC = hatmathcalS - hatmathcalS^2$. So we can write
$$ A^otimes n otimes B = (hatmathcalS + hatmathcalC)^n (A^otimes n otimes B) = sum_k=0^n binomnk (hatmathcalS^n-k hatmathcalC^k) (A^otimes n otimes B)$$
and finally applying $mu$ on both sides gives the formula you are after.
answered Mar 27 at 4:32
JoppyJoppy
5,963521
5,963521
$begingroup$
very nice, I didn't realize you could so simply define the operators via "rotations" of the character strings. Still I wonder, is there any kind of generality in this sort of approach to derive formulas? For example, I realised that one can also easily derive the explicit formula for the repeated commutator $operatornamead^k(A)B$ as $operatornamead^k(A)B=(A_L-A_R)^k B$, where $A_L, A_R$ denote "multiplication by A on the left/right". This trick can be similarly understood as operators in the tensor algebra of $V$. I wonder if something similar could work to derive, say, the BCH formula
$endgroup$
– glS
Mar 27 at 11:38
1
$begingroup$
I guess something swept under the rug in this answer was that you could actually define those operators on the tensor power. (It wasn’t obvious to me that I could make up a $hatmathcalC$ which lifted yours). Also, the fact that everything was an operator on the same tensor power. If you have to move between different tensor powers, things become a little more complicated and there is more to check. But as you say, using the tensor algebra would be the right replacement.
$endgroup$
– Joppy
Mar 27 at 11:42
add a comment |
$begingroup$
very nice, I didn't realize you could so simply define the operators via "rotations" of the character strings. Still I wonder, is there any kind of generality in this sort of approach to derive formulas? For example, I realised that one can also easily derive the explicit formula for the repeated commutator $operatornamead^k(A)B$ as $operatornamead^k(A)B=(A_L-A_R)^k B$, where $A_L, A_R$ denote "multiplication by A on the left/right". This trick can be similarly understood as operators in the tensor algebra of $V$. I wonder if something similar could work to derive, say, the BCH formula
$endgroup$
– glS
Mar 27 at 11:38
1
$begingroup$
I guess something swept under the rug in this answer was that you could actually define those operators on the tensor power. (It wasn’t obvious to me that I could make up a $hatmathcalC$ which lifted yours). Also, the fact that everything was an operator on the same tensor power. If you have to move between different tensor powers, things become a little more complicated and there is more to check. But as you say, using the tensor algebra would be the right replacement.
$endgroup$
– Joppy
Mar 27 at 11:42
$begingroup$
very nice, I didn't realize you could so simply define the operators via "rotations" of the character strings. Still I wonder, is there any kind of generality in this sort of approach to derive formulas? For example, I realised that one can also easily derive the explicit formula for the repeated commutator $operatornamead^k(A)B$ as $operatornamead^k(A)B=(A_L-A_R)^k B$, where $A_L, A_R$ denote "multiplication by A on the left/right". This trick can be similarly understood as operators in the tensor algebra of $V$. I wonder if something similar could work to derive, say, the BCH formula
$endgroup$
– glS
Mar 27 at 11:38
$begingroup$
very nice, I didn't realize you could so simply define the operators via "rotations" of the character strings. Still I wonder, is there any kind of generality in this sort of approach to derive formulas? For example, I realised that one can also easily derive the explicit formula for the repeated commutator $operatornamead^k(A)B$ as $operatornamead^k(A)B=(A_L-A_R)^k B$, where $A_L, A_R$ denote "multiplication by A on the left/right". This trick can be similarly understood as operators in the tensor algebra of $V$. I wonder if something similar could work to derive, say, the BCH formula
$endgroup$
– glS
Mar 27 at 11:38
1
1
$begingroup$
I guess something swept under the rug in this answer was that you could actually define those operators on the tensor power. (It wasn’t obvious to me that I could make up a $hatmathcalC$ which lifted yours). Also, the fact that everything was an operator on the same tensor power. If you have to move between different tensor powers, things become a little more complicated and there is more to check. But as you say, using the tensor algebra would be the right replacement.
$endgroup$
– Joppy
Mar 27 at 11:42
$begingroup$
I guess something swept under the rug in this answer was that you could actually define those operators on the tensor power. (It wasn’t obvious to me that I could make up a $hatmathcalC$ which lifted yours). Also, the fact that everything was an operator on the same tensor power. If you have to move between different tensor powers, things become a little more complicated and there is more to check. But as you say, using the tensor algebra would be the right replacement.
$endgroup$
– Joppy
Mar 27 at 11:42
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%2f3163565%2fwhy-does-this-trick-to-derive-the-formula-for-an-b-in-terms-of-repeated-com%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
$begingroup$
First note that your trick does use recursion, in the proof of the binomial theorem, so you didn't actually get rid of the recursion, only pushed it further away from sight. Secondly, I don't know how general you're trying to see things, but your trick can probably be formalized using some free noncommutative algebra on $A,B,D$
$endgroup$
– Max
Mar 26 at 19:06
$begingroup$
@Max yes ok, I didn't mean to say that I wanted a way to completely get rid of recursion for its own sake, the point was more about having a more direct way to get to the result (but also, do you really need recursion to prove the binomial theorem? I would have said that a combinatorial argument is more than sufficient for that. Then again, recursion might be used for the combinatorial formulas I guess). I am not familiar with "free algebras", but from a quick read on wikipedia it seems to be kind of a way to formalize acting on strings of "formal objects", so it seems on point.
$endgroup$
– glS
Mar 26 at 19:12