A102478 - OEIS

%I #10 Jun 27 2015 12:45:51

%S 21,39,51,54,90,96,135,150,156,165,171,195,210,261,270,306,330,411,

%T 420,441,501,570,615,636,741,771,816,885,1050,1095,1341,1371,1536,

%U 1599,1704,1821,1914,2121,2226,2286,2370,2394,2499,2811,2859,2916,3051,3129,3714

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

%H Harvey P. Dale, <a href="/A102478/b102478.txt">Table of n, a(n) for n = 1..8000</a>

%F a(n) = A068700(n) / 2. - _Reinhard Zumkeller_, Jun 27 2015

%e 2*21=42, 4241 and 4243 are twin primes so a(1)=21

%e 2*39=78, 7877 and 7879 are twin primes so a(2)=39

%t 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 *)

%o (Haskell)

%o a102478 = flip div 2 . a068700  -- _Reinhard Zumkeller_, Jun 27 2015

%Y Cf. A068700.

%K nonn,base

%O 1,1

%A _Pierre CAMI_, Feb 24 2005

%E Edited by _Charles R Greathouse IV_, Apr 29 2010