1,2

Table of n, a(n) for n=1..24.

1 is a term since 20^1 - 3 = 17 is a prime.

Do[ If[ PrimeQ[ 20^n - 3], Print[n]], {n, 0, 200000}]

(PARI) is(n)=isprime(20^n - 3)

Cf. A339921.

Sequence in context: A286991 A029905 A057437 * A196255 A024666 A184397

Adjacent sequences: A339919 A339920 A339921 * A339923 A339924 A339925

nonn,more

Paul Bourdelais, Dec 23 2020

a(24) from Paul Bourdelais, Jan 28 2021

approved