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 A178220 #13 Mar 30 2012 17:26:35
%S A178220 3,1,3,1,3,1,3,1,3,3,1,3,9,1,9,23,3,3,3,9,3,7,9,23,1,3,1,11,1,11,7,9,
%T A178220 3,9,3,1,1,7,29,3,11,1,3,1,3,3,1,7,3,3,3,3,1,11,9,3,3,1,7,9,3,9,9,9,7,
%U A178220 21,3,1,21,1,3,3,1,3,3,3,17,19,3,1,11,1,17,7,1,51,3,37,33,7
%N A178220 Smallest number that appending to n-th prime gives another prime.
%F A178220 a(n) = A030670(n) after deleting n-th prime.
%e A178220 n=1: prime(1)=2, A030670(1)=23, after deleting 2 we get a(1)=3
%e A178220 n=2: prime(2)=3, A030670(2)=31, after deleting 3 we get a(2)=1
%t A178220 f[n_] := (k = 1; tsn = ToString[n]; While[a = ToExpression[tsn <> ToString[k]]; !PrimeQ[a], k++]; k);
%t A178220 Table[f[Prime[n]], {n, 1, 200}]
%Y A178220 Cf. A030670.
%K A178220 nonn,base
%O A178220 1,1
%A A178220 _Zak Seidov_, Dec 20 2010