[ exp(17/21)*n! ].

Terms

a(0) =2a(1) =4a(2) =13a(3) =53a(4) =269a(5) =1617a(6) =11324a(7) =90592a(8) =815332a(9) =8153325a(10) =89686575a(11) =1076238905a(12) =13991105771a(13) =195875480803