A373772 Expansion of e.g.f. exp(x^3 / (6 * (1 - x))) / (1 - x).
1, 1, 2, 7, 32, 180, 1210, 9450, 84000, 836920, 9234400, 111742400, 1471023400, 20925905000, 319830310800, 5226116295400, 90906373958400, 1676967192700800, 32697692264036800, 671856896755844800, 14509136903381120000, 328520930667097168000
Offset: 0
Keywords
Programs
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PARI
a(n) = n!*sum(k=0, n\3, binomial(n-2*k, n-3*k)/(6^k*k!));
Formula
a(n) = n! * Sum_{k=0..floor(n/3)} binomial(n-2*k,n-3*k)/(6^k * k!).