a(n) = Sum_{k=0..n} (2*k)!.A138525a(n) = Sum_{k=0..n} (2*k)!.Terms a(0) =1a(1) =3a(2) =27a(3) =747a(4) =41067a(5) =3669867a(6) =482671467a(7) =87660962667External referencesoeis: A138525