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:<m>[a^\dagger a,a^\dagger b]=a^\dagger b\,.</m> | :<m>[a^\dagger a,a^\dagger b]=a^\dagger b\,.</m> | ||
+ | |||
+ | Here is a module that does it directly, computing <m>:[elem_1,elem_2]:</m>: | ||
+ | |||
+ | <pre> | ||
+ | NormalOrderCommutator[elem1_, elem2_] := Module[{}, | ||
+ | NormalOrder[{prod[elem1, elem2], prod[{1, -1}*elem2, elem1]}] | ||
+ | ] | ||
+ | </pre> | ||
+ | |||
+ | in term of another module, prod, which is a component of the NormalOrder module: | ||
+ | |||
+ | <pre> | ||
+ | prod[sequence__] := Module[{terms}, | ||
+ | terms = Transpose[List[sequence]]; | ||
+ | {Associate[Flatten[EvenSize /@ (terms[[1]])]], Times @@ terms[[2]]} | ||
+ | ] | ||
+ | |||
+ | EvenSize[list_] := Module[{}, | ||
+ | If[OddQ[Length[list]], Prepend[list, 0], list] | ||
+ | ] | ||
+ | </pre> | ||
+ | |||
+ | Some examples: | ||
+ | |||
+ | <pre> | ||
+ | NormalOrderCommutator[{{1, 1}, 1}, {{1, 0}, 1}] | ||
+ | {{{1, 0}, 1}} | ||
+ | </pre> | ||
+ | |||
+ | already given above, or | ||
+ | |||
+ | <pre> | ||
+ | NormalOrderCommutator[{{1}, 1}, {{1, 0}, 1}] | ||
+ | {{{0, 0}, 1}} | ||
+ | </pre> | ||
+ | |||
+ | which is the fundamental relation <m>[a,a^\dagger]=1</m>. |
In this note I give some particular cases of the module NormalOrder (see here).
If <m>C</m> commute with <m>A</m> and <m>B</m> then:
So it is easy to compute expressions like, e.g.,
factoring out b and computing:
NormalOrder[{{{1, 1, 1, 0}, 1}, {{2, 1}, -1}}] {{{1, 0}, 1}}
we find:
Here is a module that does it directly, computing <m>:[elem_1,elem_2]:</m>:
NormalOrderCommutator[elem1_, elem2_] := Module[{}, NormalOrder[{prod[elem1, elem2], prod[{1, -1}*elem2, elem1]}] ]
in term of another module, prod, which is a component of the NormalOrder module:
prod[sequence__] := Module[{terms}, terms = Transpose[List[sequence]]; {Associate[Flatten[EvenSize /@ (terms[[1]])]], Times @@ terms[[2]]} ] EvenSize[list_] := Module[{}, If[OddQ[Length[list]], Prepend[list, 0], list] ]
Some examples:
NormalOrderCommutator[{{1, 1}, 1}, {{1, 0}, 1}] {{{1, 0}, 1}}
already given above, or
NormalOrderCommutator[{{1}, 1}, {{1, 0}, 1}] {{{0, 0}, 1}}
which is the fundamental relation <m>[a,a^\dagger]=1</m>.