OFFSET
1,1
LINKS
Hugo Pfoertner, Table of n, a(n) for n = 1..500
EXAMPLE
a(1) = 12, because none of the 11 numbers {2^1*3^11+1, 2^2*3^10+1, ..., 2^11*3^1+1} = {354295, 236197, 157465, 104977, 69985, 46657, 31105, 20737, 13825, 9217, 6145} is prime,
a(2) = 24: none of the 23 numbers {2^1*3^23+1, 2^2*3^22+1, ..., 2^23*3^1+1} = {188286357655, 125524238437, 83682825625, 55788550417, ..., 56623105, 37748737, 25165825} is prime.
PROG
(PARI) for(s=2, 500, my(t=1); for(j=1, s-1, my(k=s-j); if(isprime(2^j*3^k+1), t=0; break)); if(t, print1(s, ", ")))
CROSSREFS
KEYWORD
nonn
AUTHOR
Hugo Pfoertner, based on a suggestion from Rainer Rosenthal, Aug 24 2020
STATUS
approved