OFFSET
1,1
COMMENTS
Primes in the sequence: 5, 7, 19, 67, 4099, 65539, 262147, 268435459, 1073741827, ...
On the other hand, for example, 2^(p-1) + 3 is composite when p == 11 (mod 12) or p == 5 (mod 18), with p>5; or when p is of the form 2*h^2+2*h*(k+2)+3*k, with k>0 and h>1.
LINKS
Vincenzo Librandi, Table of n, a(n) for n = 1..200
MATHEMATICA
Table[2^(Prime[n] - 1) + 3, {n, 25}]
PROG
(Magma) [2^(p-1)+3: p in PrimesUpTo(80)];
CROSSREFS
KEYWORD
nonn
AUTHOR
Vincenzo Librandi, Sep 17 2013
STATUS
approved