A174166 Composites n such that 2*n-1 is prime.

4, 6, 9, 10, 12, 15, 16, 21, 22, 24, 27, 30, 34, 36, 40, 42, 45, 49, 51, 52, 54, 55, 57, 64, 66, 69, 70, 75, 76, 82, 84, 87, 90, 91, 96, 99, 100, 106, 112, 114, 115, 117, 120, 121, 126, 129, 132, 135, 136, 141, 142, 147, 154, 156, 159, 166, 169, 174, 175, 177, 180

Offset: 1

Author

Juri-Stepan Gerasimov, Mar 10 2010

Keywords

• nonn

Examples

			a(1)=4 because 2*4-1=7=prime; a(2)=6 because 2*6-1=11=prime.
		

Links

• Harvey P. Dale, Table of n, a(n) for n = 1..1000

Crossrefs

Cf. A002808.

Programs

Mathematica

Select[(#+1)/2&/@Prime[Range[2,100]],CompositeQ] (* Harvey P. Dale, May 21 2023 *)

Extensions

Entries checked by R. J. Mathar, Apr 26 2010