[ exp(2/21)*n! ].

Terms

a(0) =1a(1) =2a(2) =6a(3) =26a(4) =131a(5) =791a(6) =5543a(7) =44348a(8) =399139a(9) =3991392a(10) =43905314a(11) =526863779a(12) =6849229138a(13) =95889207938