E.g.f.: exp(4x)/(1-x).

Terms

a(0) =1a(1) =5a(2) =26a(3) =142a(4) =824a(5) =5144a(6) =34960a(7) =261104a(8) =2154368a(9) =19651456a(10) =197563136a(11) =2177388800a(12) =26145442816a(13) =339957865472