A111962 Expansion of 1/(sqrt(1-2x-3x^2)-x^2).
1, 1, 4, 9, 27, 74, 215, 619, 1808, 5293, 15579, 45986, 136141, 403937, 1200854, 3575835, 10663105, 31836508, 95156251, 284684303, 852427700, 2554346249, 7659427811, 22981483554, 68992151795, 207223579671, 622697586944
Offset: 0
Formula
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}}
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
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