[ exp(8/21)*n! ].

Terms

a(0) =1a(1) =2a(2) =8a(3) =35a(4) =175a(5) =1053a(6) =7376a(7) =59015a(8) =531139a(9) =5311394a(10) =58425338a(11) =701104058a(12) =9114352757a(13) =127600938607