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 A121852 #6 Mar 20 2015 18:26:56
%S A121852 1,4,6,16,24,104,121,283,326,489,1387,1394,1424,1435,1985,4323,5221
%N A121852 Numbers n such that partial sum of Catalan numbers A014137(n) is a prime.
%C A121852 Corresponding primes are A014137( a(n)) = {2,23,197,48760367,1757900019101,...}.
%e A121852 Catalan numbers A000108(n) begin {1,1,2,5,14,42,132,429,1430,4862,...}.
%e A121852 a(1) = 1 because A014137(1) = 1 + 1 = 2 is prime,
%e A121852 a(2) = 4 because A014137(4) = 1 + 1 + 2 + 5 + 14 = 23 is prime.
%t A121852 s = 1; Do[s = s + (2n)!/((n!)^2(n + 1)); If[PrimeQ@s, Print@n], {n, 17925}] (* _Robert G. Wilson v_ *)
%Y A121852 Cf. A000108, A014137.
%K A121852 hard,nonn
%O A121852 1,2
%A A121852 _Alexander Adamchuk_, Aug 30 2006
%E A121852 a(11) - a(17) from _Robert G. Wilson v_, Aug 31 2006