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%I #7 Nov 21 2013 12:48:43
%S 677,8779,27773,67777,8777779,27777777773,67777777777,67777777777777,
%T 67777777777777777777777777,87777777777777777777777777777779,
%U 8777777777777777777777777777777777779
%N Primes such that the outer 2 digits are n and n+1 and all inner digits are 7, where 0 < n < 9.
%C Because prime numbers must end in odd digits other than 5, the only beginning/end digits combinations that satisfy the definition are (2,3), (6,7), and (8,9). [From Harvey P. Dale, Jan 29 2012]
%F a(x) = 10^(x+1)*n+floor(10^x*7/9)*10+n+1. Output if a(x) is prime.
%t Select[Flatten[Table[{FromDigits[Join[PadRight[{2},n,7],{3}]], FromDigits[ Join[PadRight[{6},n,7],{7}]],FromDigits[ Join[PadRight[ {8},n,7],{9}]]},{n,2,70}]],PrimeQ] (* _Harvey P. Dale_, Jan 29 2012 *)
%o (PARI) n10np1(n) = { local(x,y,k); for(x=1,n, for(k=1,8, y=10^(x+1)*k+floor(10^x*7/9)*10+k+1; if(isprime(y),print1(y",")) ) ) }
%K easy,nonn,base
%O 1,1
%A _Cino Hilliard_, Jul 11 2005
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