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Cauchy kernel through bidegree three
Example
Let be the part of the Cauchy kernel in bidegrees with . Its complete–monomial and Schur expansions are computed below. Pairing the -factor with gives .
Facts & Assumptions
Given: The degreewise stable ring, the finite complete and monomial conventions, their stable bases, the Cauchy expansions, the Hall form, and Jacobi–Trudi.
In the bidegree completion, ; both sums are taken by diagonal bidegree (Power-sum, complete, and Schur expansions of the Cauchy kernel).
Each is the inverse limit of the rank- homogeneous symmetric-polynomial parts, and multiplication is induced by rankwise polynomial multiplication (The stable graded ring of symmetric functions).
In rank , is the sum of all monomials of total degree , with (Power sums and complete homogeneous symmetric polynomials ).
In rank , is the sum of the distinct monomials whose exponent tuples are permutations of the padded tuple (Monomial symmetric polynomials indexed by partitions).
In rank , the indexed by partitions of length at most form a -basis of the symmetric polynomials (Monomial symmetric polynomials form an -basis of the symmetric-polynomial ring).
The stable orbit sums , for , form a -basis of and project to the finite orbit sums whenever (The monomial symmetric functions form the integral stable basis).
The finite specialize compatibly to stable elements, and denotes their stable product (Elementary and complete families freely generate the stable ring).
The Hall form is graded and satisfies (The Hall inner product on symmetric functions).
For , , with , negative subscripts zero, and the empty determinant equal to (Jacobi–Trudi and dual Jacobi–Trudi identities).
The stable Schur functions form an orthonormal basis for the Hall form: (Schur functions form an orthonormal integral basis).
The completed tensor product has diagonal bidegree kernel (Bidegree completion of two symmetric-function rings).
Verification
At rank , the projection is an isomorphism for each : for , every partition of has length at most , and [F5] and [F6] identify the stable and finite monomial bases; for , both components are with basis . Thus finite rank- coefficient calculations determine the stable coefficients in all degrees used here.
At rank , grouping monomials by distinct exponent orbits and counting ordered products gives the complete-function identities below; and , while types occur in with multiplicities .
Jacobi–Trudi evaluates the Schur functions through degree three as shown; substituting step 2.1 gives the monomial expressions, including .
The partitions of degrees are respectively , , , and , so [F1] gives these complete–monomial and Schur components of in bidegrees .
Since , duality in [F8] gives .
Similarly, .
Also, .
Gradedness removes degrees below three, so contracting the first factor in the complete–monomial expansion gives ; contraction of the Schur expansion gives the same result by [F10].
Degree gives (and [F11] specializes to when either alphabet is zero); in degree , has coefficient one. Degree is the retained upper endpoint and rank is its threshold rank; repeated parts in and have the coefficients from step 2.1, while each lists distinct monomials. All counts are finite, so no choice is used, and no equivalence is asserted.
Depends on
- Power-sum, complete, and Schur expansions of the Cauchy kernel
- The Hall inner product on symmetric functions
- Schur functions form an orthonormal integral basis
- Jacobi–Trudi and dual Jacobi–Trudi identities
- The stable graded ring of symmetric functions
- Power sums $p_k$ and complete homogeneous symmetric polynomials $h_k$
- Monomial symmetric polynomials indexed by partitions
- Monomial symmetric polynomials form an $R$-basis of the symmetric-polynomial ring
- The monomial symmetric functions form the integral stable basis
- Elementary and complete families freely generate the stable ring
- Bidegree completion of two symmetric-function rings
Used by
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Sources
- I. G. Macdonald, Symmetric Functions and Hall Polynomials, 2nd ed., Chapter I §4, equations (4.2)–(4.3), printed pp. 62–63 (standard reference, not scraped)
- Jeremy L. Martin, Lecture Notes on Algebraic Combinatorics, §9.9, printed pp. 191–195 (standard reference, not scraped)