E.g.f.: exp(3x)/(1-x).

Terms

a(0) =1a(1) =4a(2) =17a(3) =78a(4) =393a(5) =2208a(6) =13977a(7) =100026a(8) =806769a(9) =7280604a(10) =72865089a(11) =801693126a(12) =9620848953a(13) =125072630712