A378958 G.f. A(x) satisfies A(x) = 1 + x/A(x) * (1 - A(x) + A(x)^4).
1, 1, 2, 8, 32, 145, 681, 3337, 16773, 86181, 450268, 2385544, 12784861, 69189509, 377576512, 2075423744, 11480230037, 63857579629, 356962271136, 2004255583560, 11298268724556, 63919517790933, 362806671879955, 2065443363987045, 11790688867079872, 67477283970889867
Offset: 0
Keywords
Programs
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PARI
a(n) = if(n==0, 1, sum(k=0, n, (-1)^k*binomial(n, k)*binomial(3*n-4*k, n-k-1))/n);
Formula
a(n) = (1/n) * Sum_{k=0..n} (-1)^k * binomial(n,k) * binomial(3*n-4*k,n-k-1) for n > 0.