[ exp(16/21)*n! ].

Terms

a(0) =2a(1) =4a(2) =12a(3) =51a(4) =257a(5) =1542a(6) =10797a(7) =86379a(8) =777417a(9) =7774170a(10) =85515876a(11) =1026190519a(12) =13340476748a(13) =186766674472