%I #19 Apr 08 2021 07:19:25
%S 1,2,5,7,9,11,15,37,59,119,154,439,745,799,1260,1444,3306,8890,14604,
%T 15869,20195,99055,186554,396120
%N Numbers k such that 20^k - 3 is prime.
%e 1 is a term since 20^1 - 3 = 17 is a prime.
%t Do[ If[ PrimeQ[ 20^n - 3], Print[n]], {n, 0, 200000}]
%o (PARI) is(n)=isprime(20^n - 3)
%Y Cf. A339921.
%K nonn,more
%O 1,2
%A _Paul Bourdelais_, Dec 23 2020
%E a(24) from _Paul Bourdelais_, Jan 28 2021