A109274 Numbers k such that k+1 is prime, 2k+1 composite.

4, 10, 12, 16, 22, 28, 40, 42, 46, 52, 58, 60, 66, 70, 72, 82, 88, 100, 102, 106, 108, 112, 126, 130, 136, 148, 150, 162, 166, 172, 178, 180, 190, 192, 196, 222, 226, 232, 238, 240, 250, 256, 262, 268, 276, 280, 282, 292, 310, 312, 316, 346, 348, 352, 358

Offset: 1

Author

Zak Seidov, Jun 25 2005

Keywords

• nonn

Comments

Includes all terms from A109273.

Crossrefs

Cf. A109273, A109275, A307390.

Programs

Mathematica

Select[Range[358],PrimeQ[#+1]&&CompositeQ[2#+1]&] (* James C. McMahon, Mar 05 2024 *)

PARI

isok(n) = isprime(n+1) && !isprime(2*n+1); \\ Michel Marcus, Oct 06 2013

Formula

a(n) = A307390(n) - 1. - Bhavik Mehta, Aug 14 2024

Extensions

Corrected by Michel Marcus, Oct 06 2013