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where \left\{\begin{matrix} n\\k \end{matrix} \right\} are the | where \left\{\begin{matrix} n\\k \end{matrix} \right\} are the | ||
Stirling partition numbers (the number of ways to partition a set of | Stirling partition numbers (the number of ways to partition a set of | ||
− | n objects into k non-empty subsets). It is easy to obtain similar results in terms of generalized Stirling and Bell numbers for (a^k\ud{l})^n, which are results already provided by Blasiak. | + | n objects into k non-empty subsets). It is easy to obtain similar results in terms of generalized Stirling and Bell numbers for (a^k\ud{l})^n, which are results already provided by Blasiak, e.g.: |
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+ | (\ud{a}a)^n=\sum_{k=1}^nS(n,k)(\ud{a})^ka^k | ||
+ | where S(n,k) are the Stirling numbers of the second kind ([[Mathematica]] <tt>StirlingS2[n,k]</tt>). For instance: | ||
+ | (\ud{a}a)^3=\ud{a}^3a^3+3\ud{a}^2a^2+\ud{a}a\,. | ||
=== Some useful particular cases === | === Some useful particular cases === |
Contents[hide] |
Here are collected some results related with the operator a which commutation with its adjoint \ud{a} reads:
\tag{1}[a,\ud{a}]=1
(cf. Fermi algebra)
I have written a Mathematica piece of code to compute such correlators automatically. I found out in this way formulas already known in the literature, in particular from the work of Blasiak[1][2]. You can download it to play with it (check or compute results useful in your daily quantum algebra). This is detailed in this blog post.
The basic rules are:
\begin{align*} a\ket{n}&=\sqrt{n}\ket{n-1}\,,&\bra{n}\,&a=\bra{n+1}\sqrt{n+1}\,,\\ \ud{a}\ket{n}&=\sqrt{n+1}\ket{n+1}\,,&\bra{n}\,&\ud{a}=\bra{n-1}\sqrt{n}\,, \end{align*}
of which a general expression can be drawn:
a^{\dagger i}a^j a^{\dagger k}\ket{n}={(n+k)!\over(n+k-j)!}\sqrt{(n+i+k-j)!\over n!}\ket{n+i+k-j}\,.
Some particular cases:
\begin{align*} \kern-1cm{(\mathrm{for}~i\le n+j)}\kern1cm a^i{\ud{a}}^j\ket{n}&={(n+j)!\over\sqrt{n!}\sqrt{(n+j-i)!}}\ket{n+j-i}\,,\\ \kern-1cm{(\mathrm{for}~i\le n)}\kern1cm a^{\dagger j}a^i\ket{n}&={\sqrt{n!}\sqrt{(n+j-i)!}\over(n-i)!}\ket{n+j-i}\,. \end{align*}
A general result, based on Wick theorem:
a^n\ud{a}^m=\sum_{k=0}^{\min(n,m)}k!\binom{m}{k}\binom{n}{k}\ud{a}^{m-k}a^{n-k}
Another one, inferred from my Mathematica notebook above, that could certainly be proved by recurrence or directly from combinatorics:
(a\ud{a})^n=\sum_{k=0}^n \left\{\begin{matrix} n\\k \end{matrix} \right\} \ud{a}^ka^k
where \left\{\begin{matrix} n\\k \end{matrix} \right\} are the Stirling partition numbers (the number of ways to partition a set of n objects into k non-empty subsets). It is easy to obtain similar results in terms of generalized Stirling and Bell numbers for (a^k\ud{l})^n, which are results already provided by Blasiak, e.g.:
(\ud{a}a)^n=\sum_{k=1}^nS(n,k)(\ud{a})^ka^k
a\ud{a}^n=\ud{a}^na+n\ud{a}^{n-1}
A fairly general result (forall k, l\in\mathbb{N}, including 0):
aa^{\dagger k}a^la^\dagger=a^{\dagger(k+1)}a^{l+1}+(k+l+1)a^{\dagger k}a^l+kla^{\dagger(k-1)}a^{l-1}
I maintain a long list of Bose algebra expressions in canonic notations to assist me in my computations. You can glance at it to find particular and redundant cases of the above.
A compilation of useful results derived from Eq.~(1):
We define \hat n\equiv\ud{a}a.
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Special cases below
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