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Occupation bases and actual CCR/CAR domains
Statement
Let and be as in Symmetric and antisymmetric identical-particle Fock spaces. Define
For either statistics choice, write . If , choose the elementary tensor containing copies of each mode in the fixed order and set
If , list its occupied modes increasingly in the fixed enumeration, let be their elementary tensor and set
For , both vectors are the vacuum. These vectors form orthonormal bases and define unitary identifications and .
On the finite-support span of the bosonic basis, define
where the annihilation term is zero when . Their maximal closed extensions have domains
and . On , they satisfy , , and .
On the fermionic basis define
These operators are bounded on the full fermionic Fock space and satisfy and . 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.
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.
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.
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.
Cauchy–Schwarz: , with equality exactly for dependent pairs: the inner-product and square-sum pairings satisfy Cauchy–Schwarz.
Proof
Let . In the bosonic permutation average , each distinct product word with occupation profile occurs times out of permutations. Therefore The product words are orthonormal by [F1], so the squared norm of this average is ; the prefactor in makes its norm one. Distinct occupation profiles have disjoint word supports, hence their vectors are orthogonal.
If a fermionic list repeats a mode, [F2] makes its antisymmetrization zero; excludes these profiles. For distinct occupied modes, the permuted product words are distinct orthonormal vectors with coefficients , so . The prefactor normalizes the vector. Distinct occupied subsets have disjoint product-word support and are orthogonal.
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 and are countable and their coordinate descriptions are well defined.
On the bosonic basis, shifting the coordinate index gives and . Hence the stated weighted domains are exactly the domains on which the shifted coefficient sequences lie in . If and either shifted sequence converges in , coordinate convergence shows the limiting output has the same weighted-shift coordinates; its square summability puts 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 is a core for both shifts under the graph definition in [F3].
Product words span a dense subspace of each tensor power by [F1]. Applying the bounded projections 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 , form orthonormal bases of the Fock spaces.
For , direct action on gives and . For , 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 , without asserting that unbounded commutators are defined on all Fock space.
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 , 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 , if then and ; if 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
Symmetric and antisymmetric identical-particle Fock spaces
Permutation sector projections on the n-fold tensor power
Adjoint of a densely defined operator
Densely defined, closed and closable operators, and cores
Unbounded linear operators: domain, graph and extension
Cauchy–Schwarz: , with equality exactly for dependent pairs