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 A008912 #15 Nov 14 2018 15:49:52 %S A008912 1,3,6,7,10,12,15,18,19,21,25,27,28,33,36,37,42,45,46,48,52,55,57,60, %T A008912 61,63,66,69,73,75,78,82,87,88,90,91,96,102,105,106,108,111,117,118, %U A008912 120,123,126,127,133,135,136,141,144,145,147,150,153,160,162,165 %N A008912 Truncated triangular numbers (of form n*(n-3)/2 - k^2+k*n+1 for 1<=k<n). %H A008912 J. H. Conway and N. J. A. Sloane, Low-Dimensional Lattices VII: Coordination Sequences, Proc. Royal Soc. London, A453 (1997), 2369-2389 (<a href="http://neilsloane.com/doc/Me220.pdf">pdf</a>). %F A008912 Numbers of the form (n^2+n+k^2-k+4kn)/2, n>=1, k>=0. - _Bob Selcoe_, Nov 14 2018 %Y A008912 Cf. A008867. %K A008912 nonn %O A008912 1,2 %A A008912 _N. J. A. Sloane_, _R. K. Guy_