A373668 Expansion of e.g.f. exp(x / (1 - x^2)^3).
1, 1, 1, 19, 73, 901, 7921, 88831, 1261009, 15786793, 284515201, 4359416491, 88359404761, 1671036171949, 36734936604913, 831051144091351, 19848996799904161, 516144198653004241, 13522792578340917889, 391107276466207593283, 11295497154349628317801
Offset: 0
Keywords
Programs
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PARI
a(n) = n!*sum(k=0, n\2, binomial(3*n-5*k-1, k)/(n-2*k)!);
Formula
a(n) = n! * Sum_{k=0..floor(n/2)} binomial(3*n-5*k-1,k)/(n-2*k)!.
a(n) == 1 (mod 18).