In mathematics, a Fermat number, named after Pierre de Fermat, the first known to have studied them, is a positive integer of the form F n = 2 2 n + 1, {\displaystyle F_{n}=2^{2^{n}}+1,} where n is a non-negative integer. Wikipedia

Positive integer of the form +1 Wikidata

An integer which is one more than two raised to a power which is itself a power of two (i.e., is expressible in the form 22n+1 for some n ≥ 0); equivalently, a number that is one more than two raised to some power (is expressible as 2n+1) and is prime. Wiktionary