A379022 a(n) = Sum_{k=0..n} binomial(2*n+k-1,k) * binomial(2*n+k,n-k).
1, 4, 36, 370, 4012, 44824, 510498, 5892310, 68684540, 806715964, 9532070396, 113179713046, 1349276883346, 16140148109960, 193629588953214, 2328744593780590, 28068490664161756, 338960821947139640, 4100329281075440400, 49676100591186493156, 602654837914634224812
Offset: 0
Keywords
Programs
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PARI
a(n) = sum(k=0, n, binomial(2*n+k-1, k)*binomial(2*n+k, n-k));
Formula
a(n) = [x^n] ( (1 + x)/(1 - x - x^2) )^(2*n).