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Slater and symmetric occupation-number bases of the finite-dimensional permutation sectors

Statement

Slot convention. For ℓ≥1, Sℓ acts on {0,…,ℓ−1} as in Permutation sector projections on the n-fold tensor power. On one-based tuple positions use π^(k):=1+π(k−1) for 1≤k≤ℓ. Conjugation by k↦k−1 gives πσ^=π^∘σ^ and π−1^=π^−1; thus the supplier's tensor action becomes Uπ(v1⊗⋯⊗vℓ)=vπ^−1(1)⊗⋯⊗vπ^−1(ℓ). The parameters of Uπ and sgn⁡(π) remain canonical zero-based permutations.

Assume the Axiom of Countable Choice (The Axiom of Countable Choice (ACω)). Let H be a complex Hilbert space of finite dimension d≥1 with orthonormal basis (e1,…,ed) (Hilbert space, Orthonormal families, complete orthonormal systems and Hilbert bases). Let n≥1, and let H⊗n be the n-fold completed tensor power with H⊗0:=C, H⊗1:=H and H⊗(k+1):=H⊗k⊗^H (Hilbert tensor construction: inner product, completion and product bases), and let P±(n)∈B(H⊗n), n≥1, be the permutation projections of Permutation sector projections on the n-fold tensor power. For a multi-index i=(i1,…,in)∈{1,…,d}n write ei:=ei1⊗⋯⊗ein,∣i∣j:=#{k:ik=j} (j=1,…,d). For J⊆{1,…,d} with ∣J∣=n, written J={j1<⋯<jn}, put eJ∧:=1n!∑σ∈Snsgn⁡(σ) ejσ^(1)⊗⋯⊗ejσ^(n), and for λ∈W(n,d):={λ∈Nd:∑jλj=n} put orb⁡(λ):={c∈{1,…,d}n:∣c∣=λ} and mλ:=∑c∈orb⁡(λ)ec. Then:

(a) product basis: (ei)i∈{1,…,d}n is an orthonormal basis of H⊗n, and dim⁡H⊗n=dn;

(b) Slater basis: for every J with ∣J∣=n one has eJ∧=n! P−(n)(ej1⊗⋯⊗ejn) and the family (eJ∧)J⊆{1,…,d}, ∣J∣=n is an orthonormal family whose closed linear span is ran⁡P−(n); it is therefore an orthonormal basis of ran⁡P−(n), dim⁡ran⁡P−(n)=(dn), and ran⁡P−(n)={0} when n>d. Moreover for every multi-index i one has P−(n)(ei)=0 when two entries of i coincide, and P−(n)(ei)=sgn⁡(ρi) eJ∧n! when the entries of i are pairwise distinct with underlying set J and ρi∈Sn is the unique permutation with ik=jρi^(k);

(c) symmetric basis: for every λ∈W(n,d) the vector mλ is nonzero, the family (mλ)λ∈W(n,d) is orthogonal, Uπmλ=mλ for every π∈Sn, and for every multi-index i there is a real number γi>0 with P+(n)(ei)=γi m∣i∣; consequently ran⁡P+(n) is the linear span of (mλ), the family (mλ) is an orthogonal basis of ran⁡P+(n), and dim⁡ran⁡P+(n)=(n+d−1n);

(d) occupation operators: for j∈{1,…,d} the map Nj:H⊗n→H⊗n,Njx:=∑i∈{1,…,d}n∣i∣j ⟨x,ei⟩ ei, is a self-adjoint bounded endomorphism, the Nj commute pairwise, ∑j=1dNj=nI, and NjeJ∧=1J(j) eJ∧,Njmλ=λj mλ. Consequently both sectors are invariant under every Nj, the joint eigenspaces of (N1,…,Nd) inside ran⁡P−(n) are the lines CeJ∧ (one for each n-element subset J, with eigenvalue vector 1J), and the joint eigenspaces inside ran⁡P+(n) are the lines Cmλ (one for each λ∈W(n,d), with eigenvalue vector λ).

Facts & Assumptions

Given: The finite-dimensional complex Hilbert space H of dimension d≥1 with orthonormal basis (e1,…,ed), the tensor powers H⊗n with permutation projections P±(n), multi-indices i∈{1,…,d}n, subsets J with ∣J∣=n, and λ∈W(n,d).

[F1]

Tensor powers. For all n the tensor power carries bilinear elementary tensors v1⊗⋯⊗vn with ∥v1⊗⋯⊗vn∥=∥v1∥⋯∥vn∥ and ⟨v1⊗⋯⊗vn,w1⊗⋯⊗wn⟩=∏k⟨vk,wk⟩; for orthonormal families (fa)a∈A in H and (gb)b∈B in K with dense spans, the products fa⊗gb form an orthonormal family in H⊗^K with dense span; the algebraic tensor product is dense in each completion (Hilbert tensor construction: inner product, completion and product bases clauses (b)-(d)).

[F2]

Permutation projections. Uπ(v1⊗⋯⊗vn)=vπ^−1(1)⊗⋯⊗vπ^−1(n) defines unitaries with UπUσ=Uπσ and Uπ∗=Uπ−1; P+(n)=1n!∑π∈SnUπ and P−(n)=1n!∑π∈Snsgn⁡(π)Uπ are orthogonal projections with UπP−(n)=P−(n)Uπ=sgn⁡(π)P−(n), and P−(n)(v1⊗⋯⊗vn)=0 whenever two of the vectors vk coincide (Permutation sector projections on the n-fold tensor power clauses (a),(c),(d)).

[F3]

Determinant. For a square matrix A of size n over a commutative ring, write aab:=A(a−1,b−1) for 1≤a,b≤n. Relabeling the zero-based Leibniz formula then gives det⁡(A)=∑σ∈Snsgn⁡(σ)∏a=1naσ^(a),a; in particular det⁡(In)=1, and det⁡(A)=0 if some row of A is zero (For n≥1, the determinant over a commutative ring by the Leibniz formula, and ∣det⁡A∣ for a real matrix).

[F4]

Binomial coefficients as counts. (dk)=∣[d]k∣ for d,k∈N, so with k=n the number of n-element subsets of {1,…,d} is (dn) (The set [A]k of k-element subsets and the binomial coefficient (nk):=∣[n]k∣).

[F6]

Orthonormal expansions. For an orthonormal family (fa)a∈A in a Hilbert space whose closed linear span is the whole space, every x equals the limit of the finite partial sums ∑a∈F⟨x,fa⟩fa; for a finite complete orthonormal family this is the finite equality x=∑a⟨x,fa⟩fa (Parseval equivalences for an orthonormal family clauses 1 and 4, Orthonormal families, complete orthonormal systems and Hilbert bases).

[F7]

Self-adjointness and boundedness. A bounded endomorphism T of H⊗n is self-adjoint when ⟨Tx,y⟩=⟨x,Ty⟩ for all x,y, equivalently T∗=T for the Hilbert adjoint of The Hilbert-space adjoint of a bounded operator (A bounded linear operator between normed spaces).

Proof

technique · direct; build the product basis by induction from the tensor construction, determine the two permutation projections on basis vectors by explicit reindexing sums, evaluate the resulting orthonormality as a determinant, read the dimension counts off the index sets, and compute the occupation operators on the two resulting bases

Given: The data of the statement and [F1]-[F7].

1.1F1algebrainduction

[Product basis] We show by induction on n≥1 that (ei)i∈{1,…,d}n is an orthonormal basis of H⊗n. For n=1 this is the hypothesis. If it holds for n, then by [F1] applied to the orthonormal basis (ei)i∈{1,…,d}n of H⊗n and (ej)j∈{1,…,d} of H, the products ei⊗ej form an orthonormal family with dense span in H⊗(n+1)=H⊗n⊗^H. This family is finite, indexed by Q:={1,…,d}n×{1,…,d}. Its span is closed: if zr=∑q∈Qcr,qgq→z, where gq denotes the corresponding product vector, orthonormality gives ∥zr−zs∥2=∑q∈Q∣cr,q−cs,q∣2, so each coefficient sequence is Cauchy. Let cq be its limit in C; the same finite norm identity gives zr→∑q∈Qcqgq, hence z=∑q∈Qcqgq lies in the span. Since the span is also dense, it equals the whole tensor product. Thus the family is an orthonormal basis, its cardinality is d⋅dn=dn+1 and dim⁡H⊗(n+1)=dn+1. This proves the induction step and (a).

1.2F2algebra

[The alternating projection on basis vectors] Let i∈{1,…,d}n. By the action formula of [F2], P−(n)(ei)=1n!∑π∈Snsgn⁡(π)eiπ^−1(1)⊗⋯⊗eiπ^−1(n)=1n!∑τ∈Snsgn⁡(τ)eiτ^(1)⊗⋯⊗eiτ^(n) after the substitution τ=π−1. If two entries of i coincide, say ik=il with k≠l, then the transposition υ=(k−1  l−1) satisfies eiυτ^(1)⊗⋯=eiτ^(1)⊗⋯ for every τ and sgn⁡(υτ)=−sgn⁡(τ), so the sum pairs to zero and P−(n)(ei)=0 by [F2]. If the entries are pairwise distinct with underlying set J={j1<⋯<jn} and ik=jρi^(k), then eiτ^(1)⊗⋯=ejρiτ^(1)⊗⋯ and sgn⁡(τ)=sgn⁡(ρi)sgn⁡(ρiτ); substituting ω=ρiτ gives P−(n)(ei)=sgn⁡(ρi)n!∑ω∈Snsgn⁡(ω)ejω^(1)⊗⋯⊗ejω^(n)=sgn⁡(ρi)eJ∧n!, which is the second formula of (b).

1.3F1F2F3algebra

[Orthonormality of the Slater family] For subsets J,K with ∣J∣=∣K∣=n let A be the canonical matrix with A(a−1,b−1)=⟨eja,ekb⟩=δja,kb, and abbreviate this entry as Aab for 1≤a,b≤n. By [F1], ⟨eJ∧,eK∧⟩=1n!∑σ,π∈Snsgn⁡(σ)sgn⁡(π)∏l=1nAσ^(l),π^(l). For each fixed π substitute σ=ωπ and reindex the product by t=π^(l). Then ∏lAσ^(l),π^(l)=∏tAω^(t),t and sgn⁡(σ)sgn⁡(π)=sgn⁡(ω), so the sum over π contributes n! and the remaining sum is det⁡(A) by [F3]. For J=K, A=In and this is 1; for J≠K, some ja is absent from K, giving a zero row and determinant 0. Thus the Slater family is orthonormal.

2.1F6F7step 1.1algebra

[Occupation operators] For j∈{1,…,d} the formula of (d) defines a linear endomorphism of the finite-dimensional space H⊗n; by [F6] applied to the complete orthonormal family (ei) we have x=∑i⟨x,ei⟩ei and Njx=∑i∣i∣j⟨x,ei⟩ei for every x. Self-adjointness: for x,y∈H⊗n the expansions give ⟨Njx,y⟩=∑i∣i∣j⟨x,ei⟩⟨y,ei⟩‾=⟨x,Njy⟩ because the numbers ∣i∣j are real, so Nj is self-adjoint by [F7]; it is bounded because its range is spanned by the finitely many ei with eigenvalue at most n, and ∥Njx∥≤n∥x∥. The same computation on the basis vectors ei, on which Nj is diagonal with the real eigenvalues ∣i∣j, gives NjNkei=∣i∣k∣i∣jei=NkNjei and ∑jNjei=(∑j∣i∣j)ei=nei; since the ei form a basis by step 1.1 and both sides of each identity are linear, NjNk=NkNj and ∑j=1dNj=nI. This proves the operator statements of (d) except the two eigenvector formulas.

2.2F4step 1.1step 1.2step 1.3algebra

[Slater basis and its dimension] By step 1.2 each eJ∧=n! P−(n)(ej1⊗⋯⊗ejn) lies in ran⁡P−(n), and each P−(n)(ei) is either zero or a multiple of some eJ∧; since the ei form a basis of H⊗n by step 1.1 and P−(n) is linear, the closed linear span of (eJ∧) is exactly ran⁡P−(n) (a finite-dimensional, hence closed, subspace). By step 1.3 the family is orthonormal, so it is an orthonormal basis of ran⁡P−(n), indexed by the n-element subsets J of {1,…,d}; by [F4] there are (dn) of them, so dim⁡ran⁡P−(n)=(dn). If n>d, then two of the n entries of any i coincide, so P−(n)(ei)=0 for every basis vector ei by step 1.2, whence P−(n)=0 and ran⁡P−(n)={0}. In particular eJ∧≠0 whenever ∣J∣=n≤d. This completes (b).

2.3F1F2step 1.1algebra

[The symmetric sums] Fix λ∈W(n,d). The word c that lists 1 with multiplicity λ1, then 2 with multiplicity λ2, and so on through d has ∣c∣=λ, so orb⁡(λ)≠∅ and mλ=∑c∈orb⁡(λ)ec is a nonempty sum of orthonormal vectors of H⊗n, hence nonzero, with ∥mλ∥2=∣orb⁡(λ)∣ by the orthonormality of the product basis in step 1.1 [F1]. If λ≠μ then a word has a unique multiplicity vector, so orb⁡(λ)∩orb⁡(μ)=∅ and the inner product of mλ and mμ is the empty sum 0. For π∈Sn the map c↦π⋅c, (π⋅c)k:=cπ^−1(k), permutes orb⁡(λ) and Uπec=eπ⋅c by [F2], so Uπmλ=mλ. Finally, P+(n)ei=1n!∑π∈Sneπ⋅i has coefficients supported on orb⁡(∣i∣); for c∈orb⁡(∣i∣) the set {π:π⋅i=c} is the coset π0Stab⁡(i) for any π0 with π0⋅i=c, hence has cardinality ∣Stab⁡(i)∣≥1, and it is empty for c∉orb⁡(∣i∣); therefore P+(n)ei=γi m∣i∣ with γi:=∣Stab⁡(i)∣/n!>0. This proves all the assertions of (c) except the dimension.

3.1F5step 2.3algebra

[Symmetric basis and its dimension] By step 2.3 the mλ are nonzero, pairwise orthogonal and lie in ran⁡P+(n) (indeed mλ=γi−1P+(n)ei for any multi-index i with ∣i∣=λ, and such i exists because λ∈W(n,d)); conversely every P+(n)ei is a multiple of some mλ, so by linearity and the basis property of the ei the span of (mλ)λ∈W(n,d) is ran⁡P+(n). A pairwise orthogonal family of nonzero vectors spanning a finite-dimensional space is a basis, so dim⁡ran⁡P+(n)=∣W(n,d)∣=(n+d−1d−1)=(n+d−1n) by [F5]. This completes (c).

4.1step 2.1step 2.2step 3.1algebra

[Occupation numbers on the two bases] Every summand ec of eJ∧ has exactly one entry equal to j when j∈J and none otherwise, so ∣c∣j=1J(j) for every such c; and every c∈orb⁡(λ) has ∣c∣j=λj. Since the Nj are linear and act diagonally on the ec by the formula of step 2.1, this gives NjeJ∧=1J(j)eJ∧ and Njmλ=λjmλ, the remaining assertions of (d). Both basis families displayed in (b) and (c) therefore consist of joint eigenvectors of (N1,…,Nd), and the two maps J↦1J and λ↦λ are injective; hence the joint eigenspaces inside the two sectors are the lines spanned by the corresponding basis vectors, as asserted in (d), and each sector is invariant under every Nj.

5.1step 1.1step 1.2step 1.3step 2.2step 2.3step 3.1step 2.1step 4.1∎

[Assembly] Step 1.1 proves (a), steps 1.2, 1.3 and 2.2 prove (b), steps 2.3 and 3.1 prove (c), and steps 2.1 and 4.1 prove (d). The proof uses only the completed tensor powers, the permutation projections, the orthonormal expansions and the stated counting facts, under the Countable Choice hypothesis of [F1] and [F2]; no physical reading of the sectors or of the occupation operators is asserted.

Remarks

  • Reading of the operators. Nj counts the particles occupying the declared basis vector ej; the theorem proves the exact statements that the two permutation sectors diagonalise the family (N1,…,Nd) and that the antisymmetric sector has only the occupation values 0 and 1. The physical reading of this diagonalisation as an occupation readout of the framework belongs to The declared finite-mode model and its occupation readouts and Pauli exclusion in the declared finite-mode occupation readout, with the bosonic contrast.
  • Finite dimension. The dimension of H is finite and is part of the hypotheses; no infinite-dimensional Slater basis or occupation operator is constructed here. The open case is treated by different machinery on the Fock-space pair.

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