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 A072521 #10 Feb 26 2015 15:31:34 %S A072521 6,15,21,45,91,990,1711,365085,401856,713415,785631,1079715,1326006, %T A072521 2355535,2888406,5137615,5666661,5764710,9550635,9921285,10934826, %U A072521 19434495,21421785,23622501,42003195,46315500,82349361,146384605 %N A072521 a(1) = 6 and then the smallest triangular numbers such that sum of two neighbors is also a triangular number. %C A072521 The sequence is unbounded as a(n+1) is less than or equal to the n-th triangular number. %e A072521 45 is a term because 21 + 45 = 66, 45 + 91 = 136, and 66 and 136 are triangular numbers. %o A072521 (PARI) p=6; k=3; print1(p", "); for(n=1, 30, k=k+1; u=p+k*(k+1)/2; t=floor(sqrt(2*u)); while(u!=t*(t+1)/2, k=k+1; u=p+k*(k+1)/2; t=floor(sqrt(2*u))); p=k*(k+1)/2; print1(p", ")) %Y A072521 Cf. A072522. %K A072521 nonn %O A072521 1,1 %A A072521 _Amarnath Murthy_, Jul 31 2002 %E A072521 More terms from _Ralf Stephan_, Mar 30 2003