A250291 Numbers k such that (2^k+1)/3 is a semiprime.

29, 37, 41, 47, 49, 53, 67, 71, 73, 103, 107, 109, 139, 151, 179, 223, 229, 251, 269, 277, 311, 349, 353, 433, 457, 487, 503, 599, 601, 613, 619, 643, 739, 757, 827, 839, 1031, 1061, 1117, 1123, 1217

Offset

1,1

Comments

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

a(42) >= 1259.

Links

Table of n, a(n) for n=1..41.

Samuel Wagstaff, The Cunningham Project.

Example

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

Crossrefs

Cf. A000978, A085724, A007583.

Sequence in context: A134254 A089296 A089297 * A127956 A347038 A166088

Adjacent sequences: A250288 A250289 A250290 * A250292 A250293 A250294

Keyword

nonn,more

Author

Eric Chen, Dec 24 2014

Extensions

a(40)-a(41) from Max Alekseyev, Feb 25 2025

Status

approved