A337437 a(n) is the least prime of the form 2^j*3^k - 1, j > 0, k > 0, j + k = n. a(n) = 0 if no such prime exists.

5, 11, 23, 47, 0, 191, 383, 1151, 0, 6911, 6143, 27647, 0, 73727, 497663, 294911, 0, 786431, 17915903, 10616831, 0, 18874367, 188286357653, 169869311, 0, 39182082047, 10319560703, 4076863487, 0, 7247757311, 32614907903, 495338913791, 0, 51539607551, 1174136684543

Offset: 2

Author

Hugo Pfoertner, Aug 28 2020

Keywords

• nonn

Links

• Robert Israel, Table of n, a(n) for n = 2..2214

Crossrefs

Cf. A336773.

Programs

Maple

f:= proc(n) local k, p;
   if n mod 4 = 2 and n > 2 then return 0 fi;
   for k from 1 to n-1 do
     p:= 2^(n-k)*3^k-1;
     if isprime(p) then return p fi
   od;
   0
end proc:
map(f, [$2..40]); # Robert Israel, Sep 01 2020

PARI

for(n=2,36, my(pm=oo); for(j=1,n-1, my(k=n-j,p=2^j*3^k-1);if(isprime(p),pm=min(p,pm))); print1(if(pm==oo,0,pm),", "))

Formula

a(n) = 0 for n = 2 mod 4, n > 2.