By Francois Bergeron

Written for graduate scholars in arithmetic or non-specialist mathematicians who desire to examine the fundamentals approximately essentially the most vital present learn within the box, this booklet presents a radical, but available, creation to the topic of algebraic combinatorics. After recalling uncomplicated notions of combinatorics, illustration concept, and a few commutative algebra, the most fabric presents hyperlinks among the examine of coinvariant or diagonally coinvariant areas and the examine of Macdonald polynomials and comparable operators. this offers upward push to a lot of combinatorial questions in terms of items counted through prevalent numbers resembling the factorials, Catalan numbers, and the variety of Cayley timber or parking services. the writer bargains rules for extending the idea to different households of finite Coxeter teams, in addition to permutation teams.

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Example text

Xn )g(x) = 0, .. 7) fk (∂x1 , ∂x2 , . . , ∂xn )g(x) = 0. Another easy but useful observation is that I ⊥ is closed under derivation, since partial derivatives commute. 3 We extend our vectorial notation to differential monomials, setting ∂xa := an 1 ∂xa 1 · · · ∂xn . “berg” — 2009/4/13 — 13:55 — page 48 — #56 48 3. Invariant Theory Group Actions on Polynomials While our discussion is concentrated on the symmetric group and its action on polynomials by permutation of the variables, much can be stated in a more general setup.

Indeed, one version of the fundamental theorem of algebra states precisely that these are linearly d} spans RdSn is a direct independent. The fact that the set {mλ | λ consequence of the definition of symmetric polynomial. “berg” — 2009/4/13 — 13:55 — page 56 — #64 56 3. Invariant Theory Further examples of monomial symmetric polynomials, for x = x1 , x2 , x3 , x4 , are m5 = x51 + x52 + x53 + x54 , m41 = x41 x2 + x41 x3 + x41 x4 + x42 x3 + x42 x4 + x43 x4 + x1 x42 + x1 x43 + x1 x44 + x2 x43 + x2 x44 + x3 x44 , m32 = x31 x22 + x31 x23 + x31 x24 + x32 x23 + x32 x24 + x33 x24 + x21 x32 + x21 x33 + x21 x34 + x22 x33 + x22 x34 + x23 x34 , m311 = x31 x2 x3 + x31 x2 x4 + x31 x3 x4 + x32 x3 x4 + x1 x32 x3 + x1 x32 x4 + x1 x33 x4 + x2 x33 x4 + x1 x2 x3 3 + x1 x2 x34 + x1 x3 x34 + x2 x3 x34 , m221 = x21 x22 x3 + x21 x22 x4 + x21 x23 x4 + x22 x23 x4 + x21 x2 x23 + x21 x2 x24 + x21 x3 x24 + x22 x3 x24 + x1 x22 x23 + x1 x22 x24 + x1 x23 x24 + x2 x23 x24 , m2111 = x21 x2 x3 x4 + x1 x22 x3 x4 + x1 x2 x23 x4 + x1 x2 x3 x24 m11111 = 0.

This is also written in the form (q; q)m using the notion of the q-shifted factorial (a; q)m := (1 − a)(1 − aq) · · · (1 − aq m−1 ) 1 if m > 0, if m = 0. The next step is to mimic the usual expression for binomial coefficients in terms of factorials and define the q-binomial coefficients m k := q [m]q ! [m − k]q ! 12) simplifies to a positive integer polynomial. 13) with initial conditions m k q = 1, if k = 0 or k = m. 11) thus proving the equality. “berg” — 2009/4/13 — 13:55 — page 22 — #30 22 1. 17.

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Algebraic combinatorics and coinvariant spaces by Francois Bergeron
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