Fermat number
Numbers of the form formula_35 with "a", "b" any coprime integers, "a" &gt; "b" &gt; 0, are called generalized Fermat numbers. An odd prime "p" is a generalized Fermat number if and only if "p" is congruent to 1 (mod 4). (Here we consider only the case "n" &gt; 0, so 3 = formula_36 is not a counterexample.)