Littlewood-Richardson rule and commutativity morphism - MathOverflow most recent 30 from http://mathoverflow.net 2013-05-21T20:48:15Z http://mathoverflow.net/feeds/question/38665 http://www.creativecommons.org/licenses/by-nc/2.5/rdf http://mathoverflow.net/questions/38665/littlewood-richardson-rule-and-commutativity-morphism Littlewood-Richardson rule and commutativity morphism Sasha 2010-09-14T08:37:30Z 2010-09-14T10:19:55Z <h2>Background</h2> <p>Irreducible finite dimensional representations of the group $GL_n$ are parameterized by the highest weights, that is by nonincreasing sequences of integers $$\lambda_1 \ge \lambda_2 \ge \dots \ge \lambda_n.$$ Let us restrict to the case when $\lambda_n \ge 0$. Then one can encode a highest weight by a Young diagram with $\lambda_i$ boxes in the $i$-th row.</p> <p>The Littlewood-Richardson rule describes the decomposition of a tensor product $V^\lambda \otimes V^\mu$ into a direct sum of irreducibles. It says that the multiplicity of $V^\nu$ in the tensor product $V^\lambda \otimes V^\mu$ is equal to the number of so-called Littlewood-Richardson tableux in the skew-diagram $\nu\setminus\lambda$ of weight $\mu$. See <A HREF="http://en.wikipedia.org/wiki/Littlewood-Richardson_rule" rel="nofollow"> Littlewood-Richardson rule</A> for precise definitions.</p> <p>Note that the rule is not symmetric with respect to $\lambda$ and $\mu$!</p> <p>On the other hand, the category of representations of $GL_n$ has a commutativity morphism: it is a bifunctorial isomorphism $$c_{V,W}:V\otimes W \to W\otimes V, \qquad v\otimes w \mapsto w\otimes v.$$</p> <h2>Question</h2> <blockquote> <p>Is there a possibility to make Littlewood-Richardson rule compatible with the commutativity morphism?</p> </blockquote> <p>To be more precise, is there a way to associate with every Littlewood-Richardson tableau in a skew diagram $\nu\setminus\lambda$ of weight $\mu$ an embedding $V^\nu \to V^\lambda\otimes V^\mu$ such that the composition $V^\nu \to V^\lambda\otimes V^\mu \stackrel{c}\to V^\mu\otimes V^\lambda$ is the embedding associated to an appropriate Littlewood-Richardson tableau in a skew diagram $\nu\setminus\mu$ of weight $\lambda$?</p> <p>Let me emphasize that I am asking about the $GL_n$ case, although this question has sense for any reductive group. </p> http://mathoverflow.net/questions/38665/littlewood-richardson-rule-and-commutativity-morphism/38668#38668 Answer by Emmanuel Briand for Littlewood-Richardson rule and commutativity morphism Emmanuel Briand 2010-09-14T09:43:02Z 2010-09-14T10:19:55Z <p>Let $LR(\mu/\lambda;\nu)$ be the set of Littlewood-Richardson tableaux of shape $\mu/\lambda$ and weight $\nu$. Then there is a canonical bijection between $LR(\mu/\lambda;\nu)$ and $LR(\mu/\nu;\lambda)$, presented in a paper by Pak and Vallejo ("Fundamental Symmetry map"), in a paper by Danilov and Koshevoi ("Commutor"), and in a paper by Henriques and Kamnitz.</p> <p>Is this what you want ?</p> <p>In the paper by Pak and Vallejo actually two "Fundamental Symmetry maps" are presented. Danilov and Koshevoi show that they coincide, and that they coincide with their "commutor", and with the map defined by Henriques and Kamnitzer.</p> <p>The references:</p> <p>Igor Pak and Ernesto Vallejo.<br> Reductions of Young tableau bijections<br> SIAM J. Discrete Math. 24 (2010), no. 1, 113--145.<br> doi: 10.1137/070689784<br> (Also <a href="http://arxiv.org/abs/math/0408171" rel="nofollow">http://arxiv.org/abs/math/0408171</a>)</p> <p>V.I. Danilov and G.A. Koshevoi<br> The Robinson-Schensted-Knuth correspondence and the bijections of commutativity and associativity.<br> 2008 Izv. Math. 72 689<br> doi: 10.1070/IM2008v072n04ABEH002415</p> <p>A. Henriques and J. Kamnitzer<br> The octahedron recurrence and $gl_n$-crystals<br> Adv. Math. 206:1 (2006), 211-249 </p>