A339921 - OEIS

%I #23 Apr 08 2021 07:19:29

%S 1,5,7,13,15,18,22,61,85,208,271,898,2142,2856,4392,17171,74886,78901,

%T 271951,326600

%N Numbers k such that 20^k + 3 is prime.

%e 1 is a term since 20^1 + 3 = 23 is a prime.

%t Do[ If[ PrimeQ[ 20^n + 3], Print[n]], {n, 0, 100000}]

%o (PARI) is(n)=isprime(20^n + 3)

%Y Cf. A339922.

%K nonn,more

%O 1,2

%A _Paul Bourdelais_, Dec 23 2020

%E a(19)-a(20) correspond to probable primes discovered by _Paul Bourdelais_, Jan 06 2021