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%I #17 May 06 2021 03:16:34
%S 1,0,6,72,6690,1536000,1398496680,4165565871600,48724656010825410,
%T 1991141239554487077120,325362786100184356140612996,
%U 190695111051826003327799496771600,452459020719698368348441955010421696800
%N Constant term in the expansion of (-2 + Product_{k=1..n} (1 + x_k) + Product_{k=1..n} (1 + 1/x_k))^n.
%H Seiichi Manyama, <a href="/A328814/b328814.txt">Table of n, a(n) for n = 0..58</a>
%F a(n) = A328748(n,n+1) = Sum_{i=0..n} (-2)^(n-i)*binomial(n,i)*Sum_{j=0..i} binomial(i,j)^(n+1).
%t a[n_] := Sum[(-2)^(n-i) * Binomial[n, i] * Sum[Binomial[i, j]^(n+1), {j, 0, i}], {i, 0, n}]; Array[a, 13, 0] (* _Amiram Eldar_, May 06 2021 *)
%o (PARI) {a(n) = sum(i=0, n, (-2)^(n-i)*binomial(n, i)*sum(j=0, i, binomial(i, j)^(n+1)))}
%Y Cf. A328748, A328812, A328813.
%K nonn
%O 0,3
%A _Seiichi Manyama_, Oct 28 2019
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