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 A079521 #7 Jan 17 2019 17:21:26
%S A079521 0,1,2,3,5,4,10,16,13,6,31,47,45,25,8,105,158,145,96,41,10,343,501,
%T A079521 500,340,175,61,12,1198,1752,1673,1226,676,288,85,14,4056,5808,5898,
%U A079521 4326,2569,1205,441,113,16,14506,20868,20312,15608,9526,4836,1987,640,145,18,50350,71218,73000,55696,35448,18800,8418,3090,891,181,20
%N A079521 Triangular array related to tennis ball problem, read by rows.
%H A079521 G. C. Greubel, <a href="/A079521/b079521.txt">Rows n=0..100 of triangle, flattened</a>
%H A079521 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. (Fig. A.4).
%F A079521 Let c, d, and g be given by: c(t) = (1-sqrt(1-4*t))/(2*t), d(t) = (1-(1+ 2*t)*sqrt(1-4*t) -(1-2*t)*sqrt(1+4*t) +sqrt(1-16*t^2))/(4*t^2), and
%F A079521 g(t, r) = d(t)*(t*c(t))^r*(t*c(t)^3 + 2*r*c(t)) then the rows are calculated by the expansion of g(t,k) for n>=0, 0 <= k <= n. - _G. C. Greubel_, Jan 17 2019
%e A079521 0.
%e A079521 1, 2.
%e A079521 3, 5, 4.
%e A079521 10, 16, 13, 6.
%e A079521 31, 47, 45, 25, 8.
%e A079521 105, 158, 145, 96, 41, 10. ...
%t A079521 c[t_]:= (1-Sqrt[1-4*t])/(2*t); d[t_]:= (1-(1+2*t)*Sqrt[1-4*t]-(1- 2*t)*Sqrt[1+4*t] +Sqrt[1-16*t^2])/(4*t^2); g[t_, r_]:= d[t]*(t*c[t])^r*(t*c[t]^3 +2*r*c[t]); Table[SeriesCoefficient[Series[g[t, k], {t, 0, n}], n], {n, 0, 10}, {k, 0, n}] (* _G. C. Greubel_, Jan 17 2019 *)
%Y A079521 Leading diagonal gives A079522.
%Y A079521 Cf. A079513, A079520, A079521.
%K A079521 nonn,tabl
%O A079521 0,3
%A A079521 _N. J. A. Sloane_, Jan 22 2003
%E A079521 Terms a(28) onward added by _G. C. Greubel_, Jan 17 2019