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fix tribute
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2014-06-30-TributeAlain-worksheet.rst

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.. -*- coding: utf-8 -*-
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.. linkall
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.. _lascoux.factorization_gem:
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============================================================================================================
@@ -29,15 +31,13 @@ Let's build one of Young's natural idempotents for the Symmetric group
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It's indexed by a pair of standard tableaux, which we show here, in
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French notation of course::
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sage: Tableaux.global_options(convention="French")
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sage: Tableaux.options(convention="French")
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sage: tI.pp()
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sage: print
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sage: tJ.pp()
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9
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7 8
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4 5 6
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1 2 3
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sage: tJ.pp()
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1 2
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3 4 5
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....: for sigma in tJ.row_stabilizer())
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....:
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sage: squareI
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<html>...</html>
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() + (7,8) + (5,6) + ...
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Both pieces being large, their product is a huge linear combination of
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permutations. One can compute with it, but it's useless to even look
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at it::
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sage: idempotent = nablaJ * A.monomial(muI) * squareI
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sage: len(idempotent)
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<html>...</html>
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So Alain went onto a quest for a compact representation of this object
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that would be amenable to scrutiny and hand manipulation.
@@ -70,18 +70,18 @@ Lehmer code. The second step, typical of Alain, was to encode each
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such code as an exponent vector. This makes the idempotent into a huge
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multivariate polynomial::
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sage: P = QQ["x1,x1,x2,x3,x4,x5,x6,x7,x8,x9"]
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sage: P = QQ["x1,x2,x3,x4,x5,x6,x7,x8,x9"]
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sage: x = muI(P.gens())
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sage: def to_monomial(sigma):
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....: code = Permutation(sigma).to_lehmer_code()
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....: return prod( xi^ci for xi,ci in zip(x,code) )
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....: return prod(xi^ci for xi,ci in zip(x,code))
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sage: to_polynomial = A.module_morphism(to_monomial, codomain=P)
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sage: p = to_polynomial(idempotent)
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Here are its first 20 terms::
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sage: sum(p.monomials()[:20])
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<html>...</html>
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x1^5*x2^5*x3^3*x4^2*x5^3*x6^2*x7*x8 + ...
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So far, so good. But the gain is not that obvious.
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gem that came out::
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sage: factor(p)
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<html>...</html>
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(x8 - 1) * (x7 + 1) * (x5 + 1) * (x3 - 1) * (x2 + 1) * (x6^2 - x6 + 1) * (x4^2 + x4 + 1) * (x3^2 + 1) * (x1^2 + x1 + 1) * (-x1^3 + x4^2) * (-x2^4*x5^2 + x2^2*x5^3 + x2^4*x7 - x5^3*x7 - x2^2*x7^2 + x5*x7^2)
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Reference
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---------

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