[ exp(11/21)*n! ].

Terms

a(0) =1a(1) =3a(2) =10a(3) =40a(4) =202a(5) =1215a(6) =8509a(7) =68078a(8) =612703a(9) =6127038a(10) =67397424a(11) =808769099a(12) =10513998292a(13) =147195976093