This is a front-end for the Online Encyclopedia of Integer Sequences, made by Christian Perfect. The idea is to provide OEIS entries in non-ancient HTML, and then to think about how they're presented visually. The source code is on GitHub.
%I A102504 #7 Mar 14 2015 16:48:38
%S A102504 2092,2131,2797,3433,4126,5524,5710,6817,8383,8815,9472,114613,116329,
%T A102504 130213,206776,239389,282298,286642,306046,307180,311317,318310,
%U A102504 341386,360733,366529,377005,425665,430597,460441,475642,475660,478078,490870
%N A102504 Values of n for which the concatenations 1nn1, 3nn3, 7nn7 and 9nn9 are all primes.
%e A102504 E.g. n=2092 leads to 1209220921, 3209220923, 7209220927 and 9209220929, all of which are primes.
%t A102504 foQ[n_, o_] := Block[{id = IntegerDigits[n]}, PrimeQ[ FromDigits[ Join[{o}, id, id, {o}] ]]]; Select[ Range[500985], foQ[ #, 1] && foQ[ #, 3] && foQ[ #, 7] && foQ[ #, 9] &] (* _Robert G. Wilson v_, Jan 14 2005 *)
%Y A102504 For full sequences of integers of form 1nn1 (A100846), 3nn3 (A100896), 7nn7 (A100897), 9nn9 (A102484). For primes in these sequences: 1nn1 (A102496, A102497), 3nn3 (A102498, A102499), 7nn7 (A102500, A102501), 9nn9 (A102502, A102503).
%K A102504 easy,nonn,base
%O A102504 1,1
%A A102504 Mark Hudson (mrmarkhudson(AT)hotmail.com), Jan 13 2005