A339921 Numbers k such that 20^k + 3 is prime.
1, 5, 7, 13, 15, 18, 22, 61, 85, 208, 271, 898, 2142, 2856, 4392, 17171, 74886, 78901, 271951, 326600
Offset: 1
Examples
1 is a term since 20^1 + 3 = 23 is a prime.
Crossrefs
Cf. A339922.
Programs
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Mathematica
Do[ If[ PrimeQ[ 20^n + 3], Print[n]], {n, 0, 100000}] -
PARI
is(n)=isprime(20^n + 3)
Extensions
a(19)-a(20) correspond to probable primes discovered by Paul Bourdelais, Jan 06 2021