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 A078990 #11 Jun 13 2017 21:47:03 %S A078990 1,1,2,3,1,4,10,16,22,1,6,21,52,105,158,211,1,8,36,116,301,644,1198, %T A078990 1752,2306,1,10,55,216,678,1784,4088,8144,14506,20868,27230,1,12,78, %U A078990 360,1320,4064,10896,25872,55354,105704,183284,260864,338444,1,14,105 %N A078990 Triangle arising from (4,2) tennis ball problem, read by rows. %C A078990 Length of row n = 2n+1. Rows have been reversed. %H A078990 D. Merlini, R. Sprugnoli and M. C. Verri, <a href="http://dx.doi.org/10.1006/jcta.2002.3273">The tennis ball problem</a>, J. Combin. Theory, A 99 (2002), 307-344 (Table A.1). %e A078990 Triangle starts: %e A078990 1; %e A078990 1, 2, 3; %e A078990 1, 4, 10, 16, 22; %e A078990 1, 6, 21, 52, 105, 158, 211; %e A078990 ... %o A078990 (PARI) T(n,k)=if(k<0 || k>2*n,0,if(n<1,k==0,sum(j=0,k,(j+1)*T(n-1,k-j)))) %Y A078990 Final diagonal gives A079489. Row sums give A066357(n+1). %K A078990 tabf,nonn %O A078990 0,3 %A A078990 _N. J. A. Sloane_, Jan 20 2003