Show that the inner product $$\bex \sef{x_1\vee \cdots \vee x_k,y_1\vee \cdots\vee y_k} \eex$$ is equal to the permanent of the $k\times k$ matrix $\sex{\sef{x_i,y_j}}$.

Solution. $$\beex \bea &\quad \sef{x_1\vee \cdots \vee x_k,y_1\vee \cdots \vee y_k}\\ &=\frac{1}{k!} \sum_{\sigma,\tau} \sef{x_{\sigma(1)},y_{\tau(1)}} \cdots \sef{x_{\sigma(k)},y_{\tau(k)}}\\ &=\frac{1}{k!} \sum_{\sigma,\tau} \sef{x_1,y_{\tau(\sigma^{-1}(1))}} \cdots \sef{x_k,y_{\tau(\sigma^{-1}(k))}} \\ &=\frac{1}{k!} \sum_{\sigma}\sez{ \sum_{\tau} \sef{x_1,y_{\tau(\sigma^{-1}(1))}} \cdots \sef{x_k,y_{\tau(\sigma^{-1}(k))}}} \\ &=\frac{1}{k!} \sum_{\sigma}\per \sex{\sef{x_i,y_j}}\\ &=\per \sex{\sef{x_i,y_j}}. \eea \eeex$$

[Bhatia.Matrix Analysis.Solutions to Exercises and Problems]ExI.5.5