A100819 Composite numbers whose prime factors are twin primes.

9, 15, 21, 25, 27, 33, 35, 39, 45, 49, 51, 55, 57, 63, 65, 75, 77, 81, 85, 87, 91, 93, 95, 99, 105, 117, 119, 121, 123, 125, 129, 133, 135, 143, 145, 147, 153, 155, 165, 169, 171, 175, 177, 183, 187, 189, 195, 203, 205, 209, 213, 215, 217, 219, 221

Offset: 1

Author

Walter Carlini, Jan 06 2005; corrected Jan 22 2005

Keywords

• easy
• nonn

Examples

			221 = 13 * 17 (13 is the twin prime of 11 and 17 is the twin prime to 19).
		

Crossrefs

Cf. A077800 (twin primes).

Programs

Mathematica

nn = 100; p = Prime[Range[nn]]; t = {}; Do[If[p[[n + 1]] - p[[n]] == 2 || p[[n]] - p[[n - 1]] == 2, AppendTo[t, p[[n]]]], {n, nn - 1}]; Select[Range[2, t[[-1]]], ! PrimeQ[#] && Complement[Transpose[FactorInteger[#]][[1]], t] == {} &] (* T. D. Noe, May 21 2013 *)

Extensions

New name, typo in example fixed by Zak Seidov, May 20 2013