[ exp(5/21)*n! ].

Terms

a(0) =1a(1) =2a(2) =7a(3) =30a(4) =152a(5) =913a(6) =6394a(7) =51159a(8) =460433a(9) =4604330a(10) =50647634a(11) =607771613a(12) =7901030976a(13) =110614433666