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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.
1
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
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
KEYWORD
nonn
AUTHOR
Hugo Pfoertner, Aug 28 2020
STATUS
approved