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 A284251 #7 Mar 25 2017 20:41:55 %S A284251 1,1,2,5,11,35,106,337,1112,3741,12821,44576,156862,557580,1999117, %T A284251 7221134,26253632,95996850,352798772,1302457652,4828002125, %U A284251 17962539164,67052864821,251066424023,942692613615,3548654963966,13390078255129,50635057984917 %N A284251 Number of n-element subsets of [2n] whose sum is a triangular number. %H A284251 Alois P. Heinz, <a href="/A284251/b284251.txt">Table of n, a(n) for n = 0..100</a> %F A284251 a(n) = A284249(2n,n). %Y A284251 Cf. A281872, A284249. %K A284251 nonn %O A284251 0,3 %A A284251 _Alois P. Heinz_, Mar 23 2017