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Slater and symmetric occupation-number bases of the finite-dimensional permutation sectors
Statement
Slot convention. For , acts on as in Permutation sector projections on the n-fold tensor power. On one-based tuple positions use for . Conjugation by gives and ; thus the supplier's tensor action becomes . The parameters of and remain canonical zero-based permutations.
Assume the Axiom of Countable Choice (The Axiom of Countable Choice ()). Let be a complex Hilbert space of finite dimension with orthonormal basis (Hilbert space, Orthonormal families, complete orthonormal systems and Hilbert bases). Let , and let be the -fold completed tensor power with , and (Hilbert tensor construction: inner product, completion and product bases), and let , , be the permutation projections of Permutation sector projections on the n-fold tensor power. For a multi-index write For with , written , put and for put and . Then:
(a) product basis: is an orthonormal basis of , and ;
(b) Slater basis: for every with one has and the family is an orthonormal family whose closed linear span is ; it is therefore an orthonormal basis of , , and when . Moreover for every multi-index one has when two entries of coincide, and when the entries of are pairwise distinct with underlying set and is the unique permutation with ;
(c) symmetric basis: for every the vector is nonzero, the family is orthogonal, for every , and for every multi-index there is a real number with ; consequently is the linear span of , the family is an orthogonal basis of , and ;
(d) occupation operators: for the map is a self-adjoint bounded endomorphism, the commute pairwise, , and Consequently both sectors are invariant under every , the joint eigenspaces of inside are the lines (one for each -element subset , with eigenvalue vector ), and the joint eigenspaces inside are the lines (one for each , with eigenvalue vector ).
Facts & Assumptions
Given: The finite-dimensional complex Hilbert space of dimension with orthonormal basis , the tensor powers with permutation projections , multi-indices , subsets with , and .
Tensor powers. For all the tensor power carries bilinear elementary tensors with and ; for orthonormal families in and in with dense spans, the products form an orthonormal family in with dense span; the algebraic tensor product is dense in each completion (Hilbert tensor construction: inner product, completion and product bases clauses (b)-(d)).
Permutation projections. defines unitaries with and ; and are orthogonal projections with , and whenever two of the vectors coincide (Permutation sector projections on the n-fold tensor power clauses (a),(c),(d)).
Determinant. For a square matrix of size over a commutative ring, write for . Relabeling the zero-based Leibniz formula then gives ; in particular , and if some row of is zero (For , the determinant over a commutative ring by the Leibniz formula, and for a real matrix).
Binomial coefficients as counts. for , so with the number of -element subsets of is (The set of -element subsets and the binomial coefficient ).
Weak compositions. For and , (For the number of weak compositions of into parts is , and the number of compositions is for ); and ( for ; hence , the quotient is a natural number, and clause 3, with and ).
Orthonormal expansions. For an orthonormal family in a Hilbert space whose closed linear span is the whole space, every equals the limit of the finite partial sums ; for a finite complete orthonormal family this is the finite equality (Parseval equivalences for an orthonormal family clauses 1 and 4, Orthonormal families, complete orthonormal systems and Hilbert bases).
Self-adjointness and boundedness. A bounded endomorphism of is self-adjoint when for all , equivalently for the Hilbert adjoint of The Hilbert-space adjoint of a bounded operator (A bounded linear operator between normed spaces).
Proof
Given: The data of the statement and [F1]-[F7].
[Product basis] We show by induction on that is an orthonormal basis of . For this is the hypothesis. If it holds for , then by [F1] applied to the orthonormal basis of and of , the products form an orthonormal family with dense span in . This family is finite, indexed by . Its span is closed: if , where denotes the corresponding product vector, orthonormality gives , so each coefficient sequence is Cauchy. Let be its limit in ; the same finite norm identity gives , hence 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 and . This proves the induction step and (a).
[The alternating projection on basis vectors] Let . By the action formula of [F2], after the substitution . If two entries of coincide, say with , then the transposition satisfies for every and , so the sum pairs to zero and by [F2]. If the entries are pairwise distinct with underlying set and , then and ; substituting gives , which is the second formula of (b).
[Orthonormality of the Slater family] For subsets with let be the canonical matrix with , and abbreviate this entry as for . By [F1], . For each fixed substitute and reindex the product by . Then and , so the sum over contributes and the remaining sum is by [F3]. For , and this is ; for , some is absent from , giving a zero row and determinant . Thus the Slater family is orthonormal.
[Occupation operators] For the formula of (d) defines a linear endomorphism of the finite-dimensional space ; by [F6] applied to the complete orthonormal family we have and for every . Self-adjointness: for the expansions give because the numbers are real, so is self-adjoint by [F7]; it is bounded because its range is spanned by the finitely many with eigenvalue at most , and . The same computation on the basis vectors , on which is diagonal with the real eigenvalues , gives and ; since the form a basis by step 1.1 and both sides of each identity are linear, and . This proves the operator statements of (d) except the two eigenvector formulas.
[Slater basis and its dimension] By step 1.2 each lies in , and each is either zero or a multiple of some ; since the form a basis of by step 1.1 and is linear, the closed linear span of is exactly (a finite-dimensional, hence closed, subspace). By step 1.3 the family is orthonormal, so it is an orthonormal basis of , indexed by the -element subsets of ; by [F4] there are of them, so . If , then two of the entries of any coincide, so for every basis vector by step 1.2, whence and . In particular whenever . This completes (b).
[The symmetric sums] Fix . The word that lists with multiplicity , then with multiplicity , and so on through has , so and is a nonempty sum of orthonormal vectors of , hence nonzero, with by the orthonormality of the product basis in step 1.1 [F1]. If then a word has a unique multiplicity vector, so and the inner product of and is the empty sum . For the map , , permutes and by [F2], so . Finally, has coefficients supported on ; for the set is the coset for any with , hence has cardinality , and it is empty for ; therefore with . This proves all the assertions of (c) except the dimension.
[Symmetric basis and its dimension] By step 2.3 the are nonzero, pairwise orthogonal and lie in (indeed for any multi-index with , and such exists because ); conversely every is a multiple of some , so by linearity and the basis property of the the span of is . A pairwise orthogonal family of nonzero vectors spanning a finite-dimensional space is a basis, so by [F5]. This completes (c).
[Occupation numbers on the two bases] Every summand of has exactly one entry equal to when and none otherwise, so for every such ; and every has . Since the are linear and act diagonally on the by the formula of step 2.1, this gives and , the remaining assertions of (d). Both basis families displayed in (b) and (c) therefore consist of joint eigenvectors of , and the two maps and 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 .
[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. counts the particles occupying the declared basis vector ; the theorem proves the exact statements that the two permutation sectors diagonalise the family and that the antisymmetric sector has only the occupation values and . 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 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.
Depends on
Hilbert tensor construction: inner product, completion and product bases
Permutation sector projections on the n-fold tensor power
For , the determinant over a commutative ring by the Leibniz formula, and for a real matrix
The set of -element subsets and the binomial coefficient
for ; hence , the quotient is a natural number, and
For the number of weak compositions of into parts is , and the number of compositions is for
Parseval equivalences for an orthonormal family
Orthonormal families, complete orthonormal systems and Hilbert bases
Hilbert space
The Hilbert-space adjoint of a bounded operator
A bounded linear operator between normed spaces
The Axiom of Countable Choice ()
Dependency tree · 2 levels
Sources
- J. Binney and D. Skinner, The Physics of Quantum Mechanics, Section 10.1 printed pages 226-230 (PDF pages 234-238): antisymmetric two-fermion states, exclusion, and the expansion of a pair state over single-particle states
- D. Skinner, Principles of Quantum Mechanics (Cambridge Part II, Michaelmas 2021), Chapter 7 'Identical Particles', pp. 1-10: Slater determinants and the occupation-number description of antisymmetric and symmetric states; Internet Archive copy of the original https://www.damtp.cam.ac.uk/user/dbs26/PQM/chap7.pdf
- F. H. L. Essler, Advanced Quantum Theory (Oxford lecture notes, 2017/18), Chapter 1 Sections 1.1-1.4 pp. 3-11: occupation-number representation, Fock sectors and Slater determinants