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%I #38 Oct 27 2019 03:18:55
%S 1,1,5,19,217,1451,26041,249705,6116209,76432627,2373097921,
%T 36562658573,1374991573825,25188442156333,1112491608614933,
%U 23620069750701091,1198207214200181217,28930659427538020915,1657461085278025906081,44848606508761385855085
%N Constant term in the expansion of (1 + x_1 + x_2 + ... + x_n + 1/x_1 + 1/x_2 + ... + 1/x_n)^n.
%C a(n) is the number of n-step closed walks (from origin to origin) in n-dimensional lattice where each step changes at most one component by -1 or by +1. - _Alois P. Heinz_, Oct 26 2019
%H Seiichi Manyama, <a href="/A328716/b328716.txt">Table of n, a(n) for n = 0..398</a> (terms 0..199 from Alois P. Heinz)
%F a(n) = n! * [x^n] exp(x) * BesselI(0,2*x)^n. - _Ilya Gutkovskiy_, Oct 26 2019
%F a(n) ~ c * d^n * n^n, where d = 0.8047104059195202206625458331930618795... and c = 2.12946224998808159475495497... if n is even and c = 1.4189559976544232606562785... if n is odd. - _Vaclav Kotesovec_, Oct 27 2019
%Y Main diagonal of A328718.
%K nonn
%O 0,3
%A _Seiichi Manyama_, Oct 26 2019
%E a(7)-a(19) from _Alois P. Heinz_, Oct 26 2019
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