A337437 - OEIS

%I #7 Sep 01 2020 16:29:11

%S 5,11,23,47,0,191,383,1151,0,6911,6143,27647,0,73727,497663,294911,0,

%T 786431,17915903,10616831,0,18874367,188286357653,169869311,0,

%U 39182082047,10319560703,4076863487,0,7247757311,32614907903,495338913791,0,51539607551,1174136684543

%N 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.

%H Robert Israel, <a href="/A337437/b337437.txt">Table of n, a(n) for n = 2..2214</a>

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

%p f:= proc(n) local k, p;

%p    if n mod 4 = 2 and n > 2 then return 0 fi;

%p    for k from 1 to n-1 do

%p      p:= 2^(n-k)*3^k-1;

%p      if isprime(p) then return p fi

%p    od;

%p    0

%p end proc:

%p map(f, [$2..40]); # _Robert Israel_, Sep 01 2020

%o (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),", "))

%Y Cf. A336773.

%K nonn

%O 2,1

%A _Hugo Pfoertner_, Aug 28 2020