1,1

First unknown: 1123.

If (2^n+1)/3 is a semiprime, n must be prime or the square of a prime; the only known square of a prime in this sequence is 49.

Table of n, a(n) for n=1..39.

Samuel Wagstaff, The Cunningham Project

a(1) = 29 so (2^29+1)/3 = 178956971 = 59 * 3033169 is a semiprime.

Cf. A000978, A085724, A007583.

Sequence in context: A134254 A089296 A089297 * A127956 A347038 A166088

Adjacent sequences: A250288 A250289 A250290 * A250292 A250293 A250294

nonn

Eric Chen, Dec 24 2014

approved