3^(n-1)(n+3)/2-(n-1)/2.

Terms

a(0) =1a(1) =2a(2) =7a(3) =26a(4) =93a(5) =322a(6) =1091a(7) =3642a(8) =12025a(9) =39362a(10) =127935a(11) =413338a(12) =1328597a(13) =4251522a(14) =13551739a(15) =43046714a(16) =136314609a(17) =430467202a(18) =1355971703a(19) =4261625370a(20) =13366006861a(21) =41841412802a(22) =130754415027a(23) =407953774906