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 A080817 #7 Jun 08 2019 16:24:59 %S A080817 1,3,6,10,6,10,15,8,13,19,26,15,22,30,16,24,33,43,25,35,46,25,36,48, %T A080817 61,36,49,63,35,49,64,33,48,64,81,46,63,81,43,61,80,100,58,78,99,54, %U A080817 75,97,49,71,94,118,66,90,115,60,85,111,138,79,106,134,72,100,129,159,93,123 %N A080817 Leading diagonal of triangle in A080818. %F A080817 From _David Wasserman_, May 13 2004: (Start) %F A080817 a(n) = (ceiling(sqrt(n(n+1)/2)))^2 - n(n-1)/2. %F A080817 a(n) = A048761(A000217(n)) - A000217(n-1). (End) %Y A080817 Cf. A080818, A080819, A080820. %K A080817 nonn %O A080817 1,2 %A A080817 _Amarnath Murthy_, Mar 21 2003 %E A080817 More terms from _David Wasserman_, May 13 2004