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 A214696 #12 Jan 30 2014 15:50:08 %S A214696 3,7,13,17,4919,73789919 %N A214696 Sum of next a(n) > 1 positive triangular numbers is a triangular number. %C A214696 Two or more triangular numbers in the sum. %C A214696 Because an oblong number is twice a triangular number, this sequence also gives: the sum of next a(n) positive oblong numbers is an oblong number. %F A214696 a(n) is the smallest integer k > 1 such that (4*k^3 + 12*s*k^2 + 4*(3*s^2-1)*k)/3 + 1 is a square, where s = 1 + a(1) + a(2) + ... + a(n-1). - _Max Alekseyev_, Jan 30 2014 %e A214696 1+3+6 = 10 is a triangular number, so a(1)=3, then 10+15+21+28+36+45+55 = 210 is a triangular number, seven summands, so a(2)=7. %Y A214696 Cf. A000217, A073684, A214648. %K A214696 nonn,more %O A214696 1,1 %A A214696 _Alex Ratushnyak_, Jul 26 2012