21. Show that the polynomial $H \left( f , \prod _ { i = 1 } ^ { n } \left( X - r _ { i } \right) \right)$ is a function of positive type in dimension $N$.
21. Show that the polynomial $H \left( f , \prod _ { i = 1 } ^ { n } \left( X - r _ { i } \right) \right)$ is a function of positive type in dimension $N$.