Diagonalizing some matrices arising from Fourier transform on $S_n$. - MathOverflow most recent 30 from http://mathoverflow.net 2013-05-22T06:33:57Z http://mathoverflow.net/feeds/question/36813 http://www.creativecommons.org/licenses/by-nc/2.5/rdf http://mathoverflow.net/questions/36813/diagonalizing-some-matrices-arising-from-fourier-transform-on-s-n Diagonalizing some matrices arising from Fourier transform on $S_n$. John Jiang 2010-08-26T21:24:10Z 2010-08-30T22:52:11Z <p>Consider the function $f$ on $S_n$ which equals $1/n$ on all adjacent transpositions $(i,i+1)$, where we let $n+1 = 1$, and $0$ otherwise, and its Fourier transform $\hat{f}(\rho)$ evaluated at the irreducible representations. </p> <p>Recall the irreducible representations of $S_n$ are indexed by the set of partitions of $n$. Partitions here are written as a finite non-increasing sequence of positive integers that add up to $n$.</p> <p>When $\rho$ is the representation corresponding to the partition $(n)$, the matrix $\hat{f}(\rho)$ is simply the $1 \times 1$ matrix $[1]$. </p> <p>When $\rho$ is the representation corresponding to the partition $(n-1,1)$, the resulting matrix $\hat{f}(\rho)$ can be explicitly diagonalized, since it can be extended into a cyclic matrix on $\mathbb{R}^n$. The eigenvalues are simply $\cos \frac{2\pi k}{n}$ where $k = 1, \ldots n-1$. Therefore the spectral gap for that matrix (the smallest gap between $1$ and an eigenvalue not equal to $1$) is simply </p> <p>$$1-\cos \frac{2 \pi}{n} = 1-\cos \frac{2(n-1)\pi}{n} = \frac{2 \pi^2}{n^2} + O(\frac{1}{n^3})$$.</p> <p>the following questions are in increasing levels of difficulty and are interesting to Markov chain theorists:</p> <ol> <li>Is it true that all other eigenvalues of $\hat{f}(\rho)$ for some irreducible representation $\rho$ are strictly less than $1-\cos \frac{2 \pi}{n}$ in absolute value?</li> </ol> <p>Denote by $e_{\lambda,j}$, $j = 1, \ldots, d_\lambda$ the eigenvalues of $\hat{f}(\rho_\lambda)$, where $\rho_\lambda$ is the representation associated with the partition $\lambda$ and $d_\lambda$ is the dimension of that representation.</p> <ol> <li><p>For any fixed $k \in \mathbb{N}$, is it true that $(1-\max_j e_{\lambda,j}) \le (n-\lambda_1) \frac{2 \pi^2}{n^2} + O(\frac{1}{n^3})$, for $n-\lambda_1 \le k$? Here $\lambda_1$ denotes the longest part of the partition $\lambda$.</p></li> <li><p>If $\lambda > \lambda'$ in the sense that one can move blocks in the Ferrers diagram of $\lambda'$ in the up and right direction to obtain $\lambda$, for instance $(n-1,1) > (n-2,1,1)$, is it true that the spectral gap of $\hat{f}(\rho_\lambda)$ is smaller than that associated with $\lambda'$?</p></li> <li><p>Give an explicit formula for $e_{\lambda,j}$. This is most likely not possible. </p> <p>This question shows how hard it can be to diagonalize matrices and to understand the representation theory of $S_n$ at a practical level.</p></li> </ol>