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A068700 The concatenation of n with n-1 and n with n+1 both yield primes (twin primes).

%I A068700 #18 Jun 27 2015 12:45:08
%S A068700 42,78,102,108,180,192,270,300,312,330,342,390,420,522,540,612,660,
%T A068700 822,840,882,1002,1140,1230,1272,1482,1542,1632,1770,2100,2190,2682,
%U A068700 2742,3072,3198,3408,3642,3828,4242,4452,4572,4740,4788,4998,5622,5718,5832
%N A068700 The concatenation of n with n-1 and n with n+1 both yield primes (twin primes).
%C A068700 All terms are congruent to {0, 12, 18} mod 30. - _Zak Seidov_, Oct 24 2014
%C A068700 a(n) = 2 * A102478(n). - _Reinhard Zumkeller_, Jun 27 2015
%H A068700 Zak Seidov, <a href="/A068700/b068700.txt">Table of n, a(n) for n = 1..10000</a>
%e A068700 42 is a member as 4241 as well as 4243 are primes.
%p A068700 filter:= proc(n)
%p A068700 local d;
%p A068700 d:= ilog10(n)+1;
%p A068700 isprime(n*10^d+n-1) and isprime(n*10^d+n+1)
%p A068700 end proc:
%p A068700 select(filter, [$1..10^5]); # _Robert Israel_, Oct 24 2014
%t A068700 d[n_]:=IntegerDigits[n]; conQ[n_]:=And@@PrimeQ[FromDigits/@{Join[d[n],d[n+1]],Join[d[n],d[n-1]]}]; Select[Range[5850],conQ[#] &] (* _Jayanta Basu_, May 21 2013 *)
%o A068700 (PARI) for(n=2,200, if(isprime(n*10^ceil(log(n-1)/log(10))+n-1)*isprime(n*10^ceil(log(n+1)/log(10))+n+1)==1,print1(n,",")))
%o A068700 (Haskell)
%o A068700 import Data.List.Ordered (isect)
%o A068700 a068700 n = a068700_list !! (n-1)
%o A068700 a068700_list = isect a030457_list a054211_list
%o A068700 -- _Reinhard Zumkeller_, Jun 27 2015
%Y A068700 Common terms of A030458 and A052089.
%Y A068700 Intersection of A030457 and A054211; A102478.
%K A068700 base,nonn
%O A068700 1,1
%A A068700 _Amarnath Murthy_, Mar 04 2002
%E A068700 More terms from _Benoit Cloitre_, Mar 09 2002