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%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
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