Category Archives: Uncategorized

Polynomials and Representations XXV

Properties of the Young Symmetrizer Recall that for a filling , we have the subgroup of elements which take an element of the i-th row (resp. column) of T to the i-th row (resp. column) of T. Then: where  is the Young symmetrizer. … Continue reading

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Polynomials and Representations XXIV

Specht Modules Till now, our description of the irreps of are rather abstract. It would be helpful to have a more concrete construction of these representations – one way is via Specht modules. First write Thus if , the only common irrep between … Continue reading

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Polynomials and Representations XXIII

Power-Sum Polynomials We will describe how the character table of is related to the expansion of the power-sum symmetric polynomials in terms of monomials. Recall: where exactly since is not defined. Now each irrep of is of the form  for some … Continue reading

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Polynomials and Representations XXII

Product of Representations Recall that the Frobenius map gives an isomorphism of abelian groups: Let us compute what the product corresponds to on the RHS. For that, we take and where and Multiplication gives where is the partition obtained by sorting Next, we … Continue reading

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Polynomials and Representations XXI

We have established that all irreps of are defined over and hence any field of characteristic 0. For convenience we will fix . Twists For any group G and representation over  if is a group homomorphism, we can twist as follows: Sometimes, we also … Continue reading

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Polynomials and Representations XX

From now onwards, we will assume the base field K has characteristic 0. Example: d=3 Following the previous article, we examine the case of . We get 3 partitions: , and Let us compute for all From the previous article, we have: Since , is … Continue reading

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Polynomials and Representations XIX

Representations of the Symmetric Group Let [d] be the set {1,…,d}, and Sd be the group of bijections  From here on, we shall look at the representations of Note that this requires a good understanding of representation theory (character theory) of finite groups. To start, let … Continue reading

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Polynomials and Representations XVIII

Littlewood-Richardson Coefficients Recall that the Littlewood-Richardson coefficient satisfies: By the previous article, for any SSYT of shape ,  is the number of skew SSYT of shape whose rectification is Since this number is independent of our choice of as long as its shape is … Continue reading

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Polynomials and Representations XVII

Two Important Results In this article and the next, we will find a combinatorial way of computing the Littlewood-Richardson coefficient. The key result we have so far is that given any word w there is a unique SSYT T (called the rectification of … Continue reading

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Polynomials and Representations XVI

Here is the main problem we are trying to solve today. Word Problem Given a word let us consider disjoint subwords of which are weakly increasing. For example if , then we can pick two or three disjoint subwords as follows: For … Continue reading

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