A366049 Expansion of (1/x) * Series_Reversion( x*(1-x)^3/(1-x+x^2) ).
1, 2, 8, 39, 211, 1218, 7349, 45790, 292361, 1902834, 12577737, 84205212, 569788192, 3890728052, 26775751320, 185525538183, 1293171205833, 9061500578178, 63794947215218, 451028012126797, 3200898741338041, 22794860112273294, 162841330273522907
Offset: 0
Keywords
Programs
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PARI
a(n) = sum(k=0, n\2, binomial(n+1, k)*binomial(3*n-k+1, n-2*k))/(n+1);
Formula
a(n) = (1/(n+1)) * Sum_{k=0..floor(n/2)} binomial(n+1,k) * binomial(3*n-k+1,n-2*k).