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 A069902 #24 Sep 14 2023 18:07:04
%S A069902 1,3,3,5,5,7,7,3,5,11,11,13,13,7,5,17,17,19,19,7,11,23,23,5,13,13,7,
%T A069902 29,29,31,31,11,17,17,7,37,37,19,13,41,41,43,43,11,23,47,47,7,7,17,17,
%U A069902 53,53,11,11,19,29,59,59,61,61,31,7,13,13,67
%N A069902 Largest prime factor of n(n+1)/2, the n-th triangular number.
%C A069902 Essentially the same as A074399, which has many comments, references and links.
%F A069902 a(n) = A006530(A000217(n)).
%e A069902 A000217(9) = 9*(9+1)/2 = 45 = 3*3*5, therefore a(9) = 5.
%t A069902 PrimeFactors[n_]:=Flatten[Table[ #[[1]],{1}]&/@FactorInteger[n]]; Table[PrimeFactors[n*(n-1)/2][[ -1]],{n,2,6!}] (* _Vladimir Joseph Stephan Orlovsky_, Aug 12 2009 *)
%t A069902 (* Second program: *)
%t A069902 Array[FactorInteger[PolygonalNumber[#]][[-1, 1]] &, 66] (* _Michael De Vlieger_, Sep 14 2023 *)
%o A069902 (PARI) \\ written for a(n), n >= 2
%o A069902 a(n)=vecmax(factor(n*(n+1)/2)[,1]) \\ _M. F. Hasler_, May 02 2015
%Y A069902 Related properties of triangular numbers: A069901, A069903, A069904.
%Y A069902 Cf. A000217, A006530, A074399.
%K A069902 nonn,easy
%O A069902 1,2
%A A069902 _Reinhard Zumkeller_, Apr 10 2002
%E A069902 Edited by _Peter Munn_, Sep 14 2023