[ exp(10/21)*n! ].

Terms

a(0) =1a(1) =3a(2) =9a(3) =38a(4) =193a(5) =1159a(6) =8114a(7) =64912a(8) =584211a(9) =5842112a(10) =64263239a(11) =771158873a(12) =10025065354a(13) =140350914969