A102478 Numbers n such that the concatenations (2*n),(2*n-1) and (2*n),(2*n+1) give twin primes.

21, 39, 51, 54, 90, 96, 135, 150, 156, 165, 171, 195, 210, 261, 270, 306, 330, 411, 420, 441, 501, 570, 615, 636, 741, 771, 816, 885, 1050, 1095, 1341, 1371, 1536, 1599, 1704, 1821, 1914, 2121, 2226, 2286, 2370, 2394, 2499, 2811, 2859, 2916, 3051, 3129, 3714

Offset: 1

Author

Pierre CAMI, Feb 24 2005

Keywords

• nonn
• base

Examples

			2*21=42, 4241 and 4243 are twin primes so a(1)=21
2*39=78, 7877 and 7879 are twin primes so a(2)=39
		

Links

• Harvey P. Dale, Table of n, a(n) for n = 1..8000

Crossrefs

Cf. A068700.

Programs

Haskell

a102478 = flip div 2 . a068700  -- Reinhard Zumkeller, Jun 27 2015

Mathematica

concatQ[n_]:=Module[{idn=IntegerDigits[2n],concatidn},concatidn=FromDigits[Join[idn,idn]];And@@PrimeQ[{concatidn-1,concatidn+1}]]; Select[Range[4000],concatQ] (* Harvey P. Dale, Feb 07 2012 *)

Formula

a(n) = A068700(n) / 2. - Reinhard Zumkeller, Jun 27 2015

Extensions

Edited by Charles R Greathouse IV, Apr 29 2010