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
KEYWORD
nonn,base
AUTHOR
Pierre CAMI, Feb 24 2005
EXTENSIONS
Edited by Charles R Greathouse IV, Apr 29 2010
STATUS
approved