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

1,1

Links

Table of n, a(n) for n=1..55.

Example

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

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 *)

Crossrefs

Cf. A077800 (twin primes).

Sequence in context: A014076 A067800 A155474 * A046446 A046350 A225438

Adjacent sequences: A100816 A100817 A100818 * A100820 A100821 A100822

Keyword

easy,nonn

Author

Walter Carlini, Jan 06 2005; corrected Jan 22 2005

Extensions

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

Status

approved