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 A111962 #6 Jan 30 2020 21:29:15
%S A111962 1,1,4,9,27,74,215,619,1808,5293,15579,45986,136141,403937,1200854,
%T A111962 3575835,10663105,31836508,95156251,284684303,852427700,2554346249,
%U A111962 7659427811,22981483554,68992151795,207223579671,622697586944
%N A111962 Expansion of 1/(sqrt(1-2x-3x^2)-x^2).
%C A111962 Diagonal sums of A111960.
%F A111962 a(n)=sum{k=0..floor(n/2), sum{j=0..n-k, C(n-k, j)*C((j-1)/2, (j-k)/2)*2^(j-k)*(1+(-1)^(j-k))/2}}
%F A111962 D-finite with recurrence: n*a(n) +(-4*n+3)*a(n-1) +(-2*n+3)*a(n-2) +3*(4*n-9)*a(n-3) +(8*n-27)*a(n-4) +(2*n-3)*a(n-5) +3*(n-3)*a(n-6)=0. - _R. J. Mathar_, Jan 24 2020
%K A111962 easy,nonn
%O A111962 0,3
%A A111962 _Paul Barry_, Aug 23 2005