Math Help - Pr symbol question..

1. Pr symbol question..

i looked everywhere and i couldnt find what
is the link between Pr and fejer's kernel
what is Pr
?

$f_r(x)=\frac{1}{2\pi}\int_{-\pi}^{\pi}f(x-t)P_r(t)dt=\sum_{n=1}^{\infty}C_nr^{|n|}e^{inx}$