Representations of $GL(n)$ containing $S^kV$ - MathOverflow most recent 30 from http://mathoverflow.net 2013-05-23T15:39:54Z http://mathoverflow.net/feeds/question/118584 http://www.creativecommons.org/licenses/by-nc/2.5/rdf http://mathoverflow.net/questions/118584/representations-of-gln-containing-skv Representations of $GL(n)$ containing $S^kV$ Klim Efremenko 2013-01-10T23:04:36Z 2013-05-07T22:12:33Z <p>Let $V$ be a vector space of dimension $n$. Let $S^k V$ be a representation of $GL(n)$. I would like to know if there exists some characterization of finite dimensional $GL(n)$ modules $V_1,V_2$ such that exists $GL(n)$ mapping $T:S^k V \rightarrow V_1\otimes V_2$ such that for some $x$ matrix $T(x)$ is invertible .</p> <p>Added: Just to clarify: The mapping $T$ itself may not be invertible I am asking that $T(x)$ will be invertible in $V_1\otimes V_2$. For example the mapping $S^2 V \rightarrow V\otimes V$ is not invariable, but $T(x)$ is invartable for most $x$'s.</p> <p>I am interested for a base field $\mathbb{C}$. And of course this may happen only in case when $S^kV$ is a sub-representation of $V_1 \otimes V_2$. In fact I do not know for which irreps $V_1, V_2$ representation $V_1 \otimes V_2$ have $S^kV$ as a component. I will be happy if you can give me a reference for this question. </p> http://mathoverflow.net/questions/118584/representations-of-gln-containing-skv/118618#118618 Answer by Anton Fonarev for Representations of $GL(n)$ containing $S^kV$ Anton Fonarev 2013-01-11T11:24:42Z 2013-05-07T22:12:33Z <p>Regarding the question of pairs, here is the answer (hope, I'm not mistaken).</p> <p>Let me first change the notation a little bit. Suppose, we are looking for irreps $V_1=\Sigma^{\lambda}V$ and $V_2=\Sigma^{\mu}V$ where $\lambda = (\lambda_1,\ldots,\lambda_n)$ and $\mu = (\mu_1,\ldots,\mu_n)$ resp., such that there exists an embedding $S^kV\to V_1^*\otimes V_2$.</p> <p>It's immediate that such an embedding exists iff $V_2=\Sigma^{\mu}V$ is an irreducible factor in $V_1\otimes S^kV=\Sigma^{\lambda}V\otimes S^kV$. The Littlewood-Richardson rule gives us the answer. Firstly, $k=|\mu|-|\lambda|$, where $|\lambda| = \sum_i\lambda_i$. Secondly, one has the following inequalities:</p> <ul> <li>$\mu_1\geq \lambda_1$,</li> <li>$\lambda_1\geq \mu_2 \geq \lambda_2$,</li> <li>$\ldots$</li> <li>$\lambda_{n-1}\geq \mu_n \geq \lambda_n$.</li> </ul> <p>Finally, if such an embedding exists, it's unique by the very same L-R rule.</p> <p>UPD: Forgot to mention that <strike>the main question still seems to be very hard as even</strike> checking that $\dim V_1=\dim V_2$ is a huge problem.</p> <p>UPD 2: Let me answer your main question (despite UPD). Let's forget about representation theory and do some linear algebra. What we have is a surjective map $V_1\otimes U\to V_2$, where $\dim V_1 = \dim V_2$. Let the corresponding bilinear map be $h:V_1\times U\to V_2$. We want to find such $u\in U$ that $h_u = h(-, u)$ is invertible, equivalently, of maximal rank.</p> <p>Well, this seems to be quite obvious: let $r$ be the maximal rank of $h_u$ among all $u\in U$. Suppose $r&lt;\dim V_2$. Let $u_0\in U$ be such that $\mathrm{rk}\ h_{u_0}=r$. Take any $v\in V_2\setminus\mathrm{Im}\ h_{u_0}$. Then there exists such $u_1\in U$ that $v\in \mathrm{Im}\ h_{u_1}$. Now it's easy to see that the rank of a general linear combination $\alpha h_{u_0} + \beta h_{u_1} = h_{\alpha u_0+\beta u_1}$ will be greater than $r$.</p>