[ exp(20/21)*n! ].

Terms

a(0) =2a(1) =5a(2) =15a(3) =62a(4) =311a(5) =1866a(6) =13063a(7) =104504a(8) =940539a(9) =9405390a(10) =103459293a(11) =1241511527a(12) =16139649858a(13) =225955098012