a(n) = (4*n+1)! * Sum_{k=0..n} 1 / (4*k+1)!.A337728a(n) = (4*n+1)! * Sum_{k=0..n} 1 / (4*k+1)!.Terms a(0) =1a(1) =121a(2) =365905a(3) =6278929801External referencesoeis: A337728