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 A049617 #10 Aug 05 2021 19:40:22
%S A049617 0,2,5,8,13,18,23,30,39,46,55,66,75,88,101,110,127,144,157,176,193,
%T A049617 206,227,250,267,288,313,332,357,386,403,434,467,488,521,546,571,608,
%U A049617 645,670,703,744,769,812,853,878,923,970,1003,1046,1087
%N A049617 a(n) = Sum_{i=0..2n} (-1)^i * T(i,2n-i) where T is A049615.
%p A049617 A049617 := proc(n)
%p A049617 add((-1)^i*A049615(i,2*n-i),i=0..2*n) ;
%p A049617 end proc: # _R. J. Mathar_, Oct 26 2015
%K A049617 nonn
%O A049617 0,2
%A A049617 _Clark Kimberling_