A372412 Coefficient of x^n in the expansion of ( (1-x+x^2) / (1-x) )^(3*n).
1, 0, 6, 9, 78, 225, 1293, 4851, 24174, 101583, 480531, 2123913, 9869973, 44669742, 206614827, 946394109, 4378019310, 20189406771, 93556141449, 433284753414, 2011960692003, 9345929458455, 43484293732413, 202453490369727, 943647920498997, 4401470801019600
Offset: 0
Keywords
Programs
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PARI
a(n, s=2, t=3, u=3) = sum(k=0, n\s, binomial(t*n, k)*binomial((u-t+1)*n-(s-1)*k-1, n-s*k));
Formula
a(n) = Sum_{k=0..floor(n/2)} binomial(3*n,k) * binomial(n-k-1,n-2*k).
The g.f. exp( Sum_{k>=1} a(k) * x^k/k ) has integer coefficients and equals (1/x) * Series_Reversion( x * (1-x)^3 / (1-x+x^2)^3 ). See A369230.