Euler transform of powers of 3.

Terms

a(0) =1a(1) =3a(2) =15a(3) =64a(4) =276a(5) =1137a(6) =4648a(7) =18585a(8) =73494a(9) =286834a(10) =1108470a(11) =4243128a(12) =16111333a(13) =60718488a(14) =227302086a(15) =845689753a(16) =3128786415a(17) =11515509603a(18) =42179651417a(19) =153808740042a(20) =558532554942