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Occupation bases and actual CCR/CAR domains

Statement

Let I and F±(ℓ2(I)) be as in Symmetric and antisymmetric identical-particle Fock spaces. Define

ΩB:={m:I→N0:m has finite support},ΩF:={m:I→{0,1}:m has finite support}.

For either statistics choice, write ∣m∣=∑jmj. If m∈ΩB, choose the elementary tensor xm containing mj copies of each mode j in the fixed order and set

∣m⟩B:=∣m∣!∏jmj! P+(∣m∣)xm.

If m∈ΩF, list its occupied modes increasingly in the fixed enumeration, let xm be their elementary tensor and set

∣m⟩F:=∣m∣! P−(∣m∣)xm.

For ∣m∣=0, both vectors are the vacuum. These vectors form orthonormal bases and define unitary identifications F+(ℓ2(I))≅ℓ2(ΩB) and F−(ℓ2(I))≅ℓ2(ΩF).

On the finite-support span Dfin of the bosonic basis, define

aj∣m⟩B=mj∣m−1j⟩B,aj†∣m⟩B=mj+1∣m+1j⟩B,

where the annihilation term is zero when mj=0. Their maximal closed extensions have domains

D(aj)={c:∑m∈ΩBmj∣cm∣2<∞},D(aj†)={c:∑m∈ΩB(mj+1)∣cm∣2<∞},

and aj†=aj∗. On Dfin, they satisfy [ai,aj†]=δijI, [ai,aj]=0, and [ai†,aj†]=0.

On the fermionic basis define

cj∣m⟩F=(−1)∑i<jmimj∣m−1j⟩F,cj†∣m⟩F=(−1)∑i<jmi(1−mj)∣m+1j⟩F.

These operators are bounded on the full fermionic Fock space and satisfy {ci,cj†}=δijI and {ci,cj}={ci†,cj†}=0. These are mathematical sector relations; they do not select a physical species or imply a spin-statistics theorem.

Facts & Assumptions

Given: the Fock spaces, completed tensor powers, projections, coordinate basis and fixed enumeration in Symmetric and antisymmetric identical-particle Fock spaces.

[F1]

Symmetric and antisymmetric identical-particle Fock spaces, Definition: the product tensors form a complete orthonormal family in each tensor power, the sector projections are orthogonal, and the Fock space is the square-summable direct sum of their ranges.

[F2]

Permutation sector projections on the n-fold tensor power, Statement (b)–(d): symmetrizing and antisymmetrizing by the finite permutation averages gives the indicated orthogonal sectors, and an antisymmetrized elementary tensor with a repeated factor is zero.

[F3]

Adjoint of a densely defined operator: a vector lies in the adjoint domain exactly when the adjoint pairing on the dense domain is bounded; testing on basis vectors determines its adjoint coordinates. Densely defined, closed and closable operators, and cores: a closed operator is determined by its graph, and graph-norm truncations give a core.

Proof

1.1givenF1algebra

Let n=∣m∣. In the bosonic permutation average P+(n)xm, each distinct product word with occupation profile m occurs ∏jmj! times out of n! permutations. Therefore P+(n)xm=∏jmj!n!∑w∈W(m)w,∣W(m)∣=n!∏jmj!. The product words are orthonormal by [F1], so the squared norm of this average is ∏jmj!/n!; the prefactor in ∣m⟩B makes its norm one. Distinct occupation profiles have disjoint word supports, hence their vectors are orthogonal.

1.2givenF1F2algebra

If a fermionic list repeats a mode, [F2] makes its antisymmetrization zero; ΩF excludes these profiles. For distinct occupied modes, the n! permuted product words are distinct orthonormal vectors with coefficients ±1/n!, so ∥P−(n)xm∥2=1/n!. The prefactor n! normalizes the vector. Distinct occupied subsets have disjoint product-word support and are orthogonal.

1.3givenalgebra

The fixed enumeration identifies a finite-support configuration with a finite sequence of natural indices and finitely many nonnegative integer labels (or bits); the union over all finite sequence lengths is countable. Thus ΩB and ΩF are countable and their ℓ2 coordinate descriptions are well defined.

1.4givenF3F4algebra

On the bosonic basis, shifting the coordinate index gives ∥ajc∥2=∑mmj∣cm∣2 and ∥aj†c∥2=∑m(mj+1)∣cm∣2. Hence the stated weighted domains are exactly the domains on which the shifted coefficient sequences lie in ℓ2. If c(r)→c and either shifted sequence converges in ℓ2, coordinate convergence shows the limiting output has the same weighted-shift coordinates; its square summability puts c in the displayed domain. Thus both maximal shifts are closed. Testing the adjoint identity against each basis vector forces the reverse weighted shift [F3], while Cauchy--Schwarz gives the converse pairing bound [F4]. Truncating to finite initial sets of configurations makes the weighted tails tend to zero in graph norm, so Dfin is a core for both shifts under the graph definition in [F3].

2.1step 1.1step 1.2F1algebra

Product words span a dense subspace of each tensor power by [F1]. Applying the bounded projections P±(n) to that dense span gives a dense subspace of each projected sector. Every projected product word is either zero or a scalar multiple of the corresponding occupation vector from steps 1.1–1.2. Thus the occupation vectors are complete in each sector and, after taking the direct sum over n, form orthonormal bases of the Fock spaces.

2.2step 1.4algebra

For i=j, direct action on ∣m⟩B gives ajaj†∣m⟩B=(mj+1)∣m⟩B and aj†aj∣m⟩B=mj∣m⟩B. For i≠j, both orders of distinct coordinate shifts give the same coefficient and same occupation vector, so the shifts commute; the same direct coordinate calculation applies to two annihilators and two creators. This proves the CCR on Dfin, without asserting that unbounded commutators are defined on all Fock space.

3.1givenstep 1.2algebra∎

Each fermionic shift maps a subset of an orthonormal basis into another orthonormal subset with coefficients of modulus at most one, so it is a contraction on the finite span and extends boundedly. For distinct i,j, the two orders in any CAR anticommutator are either both zero or have equal magnitudes and opposite signs: toggling the earlier mode changes by one the number of occupied modes preceding the later index. For i=j, if mj=0 then cjcj†∣m⟩F=∣m⟩F and cj†cj∣m⟩F=0; if mj=1 the roles reverse. The other same-mode anticommutators vanish. Thus all CAR identities hold on the basis span and extend by boundedness to the whole Fock space.

Depends on

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