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

2,1

Links

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

Formula

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

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

Prog

(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), ", "))

Crossrefs

Cf. A336773.

Sequence in context: A295959 A156109 A192954 * A107010 A175942 A181669

Adjacent sequences: A337434 A337435 A337436 * A337438 A337439 A337440

Keyword

nonn

Author

Hugo Pfoertner, Aug 28 2020

Status

approved