A373719 a(n) = Sum_{k=0..floor(n^2/(2*n+1))} binomial(n * (n-2*k),k).
1, 1, 1, 4, 9, 26, 91, 281, 1105, 4105, 16576, 70643, 301405, 1382928, 6363876, 30605836, 150820769, 758835104, 3941917840, 20787546715, 112615930451, 620969188400, 3492709446326, 20034747631656, 116780977502105, 693539635192626, 4181549476945504, 25627647913369903
Offset: 0
Keywords
Programs
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PARI
a(n) = sum(k=0, n^2\(2*n+1), binomial(n*(n-2*k), k));
Formula
a(n) = [x^n] 1/(1 - x * (1 + x^2)^n).