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 A065384 #9 Jun 20 2014 07:13:57
%S A065384 3,5,7,13,19,23,31,43,53,61,73,89,103,113,131,151,167,181,199,229,251,
%T A065384 271,293,317,349,373,401,433,463,491,523,557,593,619,661,701,739,773,
%U A065384 811,859,887,941,983,1033,1069,1123,1171,1223,1259,1321,1373,1429,1483
%N A065384 Largest prime <= n * (n + 1) / 2.
%H A065384 Harry J. Smith, <a href="/A065384/b065384.txt">Table of n, a(n) for n = 2..1000</a>
%t A065384 PrimePrev[n_]:=Module[{k=n},While[ !PrimeQ[k],k-- ];k];f[n_]:=n*(n+1)/2;lst={};Do[AppendTo[lst,PrimePrev[f[n]]],{n,6!}];lst (* _Vladimir Joseph Stephan Orlovsky_, Feb 26 2010 *)
%o A065384 (PARI) { for (n=2, 1000, write("b065384.txt", n, " ", precprime(n*(n + 1)/2)) ) } [_Harry J. Smith_, Oct 17 2009]
%Y A065384 Cf. A065382, A053001, A064383, A000217.
%K A065384 nonn
%O A065384 2,1
%A A065384 _Reinhard Zumkeller_, Nov 05 2001