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Finite-mode occupation configurations: fermionic exclusion counts, bosonic counts and truncated dimensions
Statement
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), let and be as in Slater and symmetric occupation-number bases of the finite-dimensional permutation sectors (with ), and let and be the symmetric vectors of that theorem. Then:
(a) fermionic configurations: the map is a bijection from the set of -element subsets of onto the set ; consequently
(b) bosonic configurations: the assignment induces a bijection from onto the set of lines spanned by the orthogonal basis , so the number of bosonic configurations of total occupation exactly is , and for every the number of bosonic configurations of total occupation at most is
(c) exclusion counts: no fermionic configuration has an entry , so the antisymmetric Slater index set contains no with for some ; for there is no fermionic configuration at all and , while for every and every the bosonic configuration with and all other entries occurs, so no occupation bound holds in the symmetric sector;
(d) consistency with the total occupation: for every one has , and for every fermionic configuration likewise; in particular the total occupation equals the particle number in both sectors.
Facts & Assumptions
Given: The finite-dimensional complex Hilbert space of dimension with orthonormal basis , the sectors of the cited theorem, and .
Empty configuration, directly. At , the Statement's conventions give and . The empty tensor is the scalar , so the empty symmetric vector is , where is the unique element of . Thus both sector ranges are one-dimensional and have this single normalized basis vector; the unique fermionic configuration is the indicator of the empty subset and is also . These are direct calculations in , not consequences of the cited theorem.
Occupation basis for positive particle number. For , the Slater vectors , with , form an orthonormal basis of , and with for ; the symmetric vectors form an orthogonal basis of with ; each has occupation vector and each has occupation vector (Slater and symmetric occupation-number bases of the finite-dimensional permutation sectors clauses (b),(c),(d)).
Binomial counts and symmetry. is the number of -element subsets of , , and for (The set of -element subsets and the binomial coefficient ); also ( for ; hence , the quotient is a natural number, and clause 3, with and ).
Weak compositions. For and , (For the number of weak compositions of into parts is , and the number of compositions is for ).
Pascal rule and hockey stick. For all , and as an -valued finite sum (Pascal's rule , and the hockey-stick identity ); the sector formula of the theorem is stated for the finite sums there defined.
Row sum. in (, and for clause 1).
Proof
Given: The data of the statement and [F0]-[F5].
[Fermionic configurations] Let have ; its indicator lies in with . Conversely, if has , then satisfies and . The two assignments are inverse bijections, so the number of fermionic configurations of total occupation equals the number of -element subsets, which is by [F2]. For , this is by [F1]; for , the dimension is by [F0] and the boundary value in [F2]. This proves (a)'s dimension assertion in all cases.
[Bosonic configurations] If , then and by [F0], so its assignment is a bijection onto the single basis line of . If , the assignment is injective by the linear independence of the orthogonal family [F1], and its image spans , so it is a bijection onto the set of basis lines. In either case, the number of its elements is by [F3] and [F2], the first assertion of (b).
[Truncated bosonic dimension] For the dimension is given by [F1] and [F2]; at it is by [F0] and the boundary value of the binomial coefficient. Thus . Substituting , the terms with vanish by the boundary values of the binomial coefficient, so the hockey stick [F4] with and upper index gives ; this completes (b).
[Fermionic row sum] Summing the dimensions over all particle numbers, using [F0] at and [F1] for and the vanishing for , gives by the row sum [F5], which completes (a).
[Exclusion counts] By construction every fermionic configuration lies in , so no entry is ; for the set is empty because at most entries can be equal to , and then by [F1], while for the bosonic sector the configuration with and all other entries lies in for every , so no occupation bound holds there; this is (c).
[Consistency and assembly] Each satisfies by definition, and each fermionic configuration in does so as well, which is (d); step 1.1 and 2.1 prove (a), steps 1.2 and 1.3 prove (b), and step 2.2 proves (c). Only the cited spectral basis and finite-counting facts were used.
Remarks
- Reading. The theorem is the finite-mode bookkeeping behind the occupation-number description: for a -mode one-particle space there are antisymmetric -particle sector dimensions and symmetric ones, and the antisymmetric configurations have all occupations in while the symmetric ones do not. The physical reading of the occupations as a readout of the framework is fixed on The declared finite-mode model and its occupation readouts and Pauli exclusion in the declared finite-mode occupation readout, with the bosonic contrast.
- Truncation. The sums over are finite sums of finite dimensions; no infinite Fock space and no convergence statement is made here.
Depends on
Slater and symmetric occupation-number bases of the finite-dimensional permutation sectors
Permutation sector projections on the n-fold tensor power
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
Pascal's rule , and the hockey-stick identity
, and for
Hilbert space
The Axiom of Countable Choice ()
Dependency tree · 2 levels
Sources
- F. H. L. Essler, Advanced Quantum Theory (Oxford lecture notes, 2017/18), Chapter 1 Sections 1.2-1.3 pp. 5-9: occupation-number states, the two sectors and the exclusion of repeated fermionic occupations
- J. Binney and D. Skinner, The Physics of Quantum Mechanics, Section 10.1 printed pages 226-228 (PDF pages 234-236): the antisymmetric expansion of a pair state and the vanishing amplitude of a repeated single-particle state
- D. Skinner, Principles of Quantum Mechanics (Cambridge Part II, Michaelmas 2021), Chapter 7 'Identical Particles', pp. 4-10: Slater determinants, occupations and the filling of single-particle levels; Internet Archive copy of the original https://www.damtp.cam.ac.uk/user/dbs26/PQM/chap7.pdf