a(n) = (4*n)! * Sum_{k=0..n} 1 / (4*k)!.A337727a(n) = (4*n)! * Sum_{k=0..n} 1 / (4*k)!.Terms a(0) =1a(1) =25a(2) =42001a(3) =498971881External referencesoeis: A337727