Sum 3^max(k,n-k),k=0..n.

Terms

a(0) =1a(1) =6a(2) =21a(3) =72a(4) =225a(5) =702a(6) =2133a(7) =6480a(8) =19521a(9) =58806a(10) =176661a(11) =530712a(12) =1592865a(13) =4780782a(14) =14344533a(15) =43040160a(16) =129127041a(17) =387400806a(18) =1162222101a(19) =3486725352a(20) =10460235105a(21) =31380882462a(22) =94142824533a(23) =282429005040a(24) =847287546561