a(n) = 4^n * n!^2 * Sum_{k=0..n} 1/k!.A052140a(n) = 4^n * n!^2 * Sum_{k=0..n} 1/k!.Terms a(0) =1a(1) =8a(2) =160a(3) =6144a(4) =399360a(5) =40058880a(6) =5771427840External referencesoeis: A052140