A366034 G.f. A(x) satisfies A(x) = 1/(1 - x)^3 + x*(1 - x)^3*A(x)^3.
1, 4, 15, 70, 405, 2676, 19075, 142562, 1100961, 8711968, 70257055, 575269278, 4769615773, 39961571228, 337805166747, 2877506096794, 24675158973081, 212835736433304, 1845348003175063, 16073746202176150, 140590118902532757, 1234285061013293716
Offset: 0
Keywords
Programs
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PARI
a(n) = sum(k=0, n, binomial(n+2*k+2, n-k)*binomial(3*k, k)/(2*k+1));
Formula
a(n) = Sum_{k=0..n} binomial(n+2*k+2,n-k) * binomial(3*k,k)/(2*k+1).