a(n) = Sum_{k=1..n} (2*k)!.A138524a(n) = Sum_{k=1..n} (2*k)!.Terms a(0) =2a(1) =26a(2) =746a(3) =41066a(4) =3669866a(5) =482671466a(6) =87660962666External referencesoeis: A138524