A381815 Smallest k>1 such that 10*k^(3*2^n)+1 is prime.
3, 2, 2, 2, 138, 24, 695, 107, 250, 404, 4657, 2185, 27931
Offset: 0
Examples
a(0) = 3, because 10*3^(3*2^0)+1 equals 271 which is prime. a(1) = 2, because 10*2^(3*2^1)+1 equals 641 which is prime.
Programs
-
Python
from sympy import isprime from itertools import count def a(n): return next(k for k in count(2) if isprime(k**(3*(2**n)) * 10 + 1))
Extensions
a(10)-a(11) from Michael S. Branicky, Mar 07 2025
a(12) from Georg Grasegger, Apr 15 2025