Sandbox:DavidKohler/Alon try 1/Ramanujan (functions)

Definition

A function f on the positive integers is d-Ramanujan for an integer d if there exists a polynomial p and a constant c > 0 such that f(k) ~ (d-1)^k p(k); or more precisely, if

$\left|f(k)-(d-1)^{k}p(k)\right|\leq ck^{c}(d-1)^{k/2}$