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 A102478 Numbers n such that the concatenations (2*n),(2*n-1) and (2*n),(2*n+1) give twin primes. 2
 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 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 LINKS Harvey P. Dale, Table of n, a(n) for n = 1..8000 FORMULA a(n) = A068700(n) / 2. - Reinhard Zumkeller, Jun 27 2015 EXAMPLE 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 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 *) PROG (Haskell) a102478 = flip div 2 . a068700  -- Reinhard Zumkeller, Jun 27 2015 CROSSREFS Cf. A068700. Sequence in context: A139768 A176071 A072708 * A221048 A182297 A270667 Adjacent sequences:  A102475 A102476 A102477 * A102479 A102480 A102481 KEYWORD nonn,base AUTHOR Pierre CAMI, Feb 24 2005 EXTENSIONS Edited by Charles R Greathouse IV, Apr 29 2010 STATUS approved

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Last modified March 22 01:06 EDT 2019. Contains 321406 sequences. (Running on oeis4.)