This is a front-end for the Online Encyclopedia of Integer Sequences, made by Christian Perfect. The idea is to provide OEIS entries in non-ancient HTML, and then to think about how they're presented visually. The source code is on GitHub.
%I A339921 #23 Apr 08 2021 07:19:29
%S A339921 1,5,7,13,15,18,22,61,85,208,271,898,2142,2856,4392,17171,74886,78901,
%T A339921 271951,326600
%N A339921 Numbers k such that 20^k + 3 is prime.
%e A339921 1 is a term since 20^1 + 3 = 23 is a prime.
%t A339921 Do[ If[ PrimeQ[ 20^n + 3], Print[n]], {n, 0, 100000}]
%o A339921 (PARI) is(n)=isprime(20^n + 3)
%Y A339921 Cf. A339922.
%K A339921 nonn,more
%O A339921 1,2
%A A339921 _Paul Bourdelais_, Dec 23 2020
%E A339921 a(19)-a(20) correspond to probable primes discovered by _Paul Bourdelais_, Jan 06 2021