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 A173867 #8 Sep 19 2012 18:56:48
%S A173867 3,13,41,131,421,1429,4861,16787,58771,208009,742897,2674429,9694843,
%T A173867 35357657,129644783,477638699,1767263171,6564120403,24466266973,
%U A173867 91482563633,343059613639,1289904147227,4861946401451,18367353072143
%N A173867 Largest prime < n-th Catalan number.
%t A173867 PrimePrev[n_] := Module[{k=n-1}, While[ !PrimeQ[k], k--]; k]; f[n_] := (2n)!/n!/(n+1)!; Table[PrimePrev[f[n]], {n, 40}]
%t A173867 NextPrime[#,-1]&/@CatalanNumber[Range[3,30]] (* _Harvey P. Dale_, Sep 19 2012 *)
%Y A173867 Cf. A000108.
%K A173867 nonn
%O A173867 3,1
%A A173867 _Vladimir Joseph Stephan Orlovsky_, Feb 28 2010