[ exp(1/15)*n! ].

Terms

a(0) =1a(1) =2a(2) =6a(3) =25a(4) =128a(5) =769a(6) =5387a(7) =43099a(8) =387896a(9) =3878966a(10) =42668628a(11) =512023541a(12) =6656306045a(13) =93188284635