a(n) = A137576((3^n-1)/2).

Terms

a(0) =1a(1) =3a(2) =13a(3) =55a(4) =217a(5) =811a(6) =2917a(7) =10207a(8) =34993a(9) =118099a(10) =393661a(11) =1299079a(12) =4251529a(13) =13817467a(14) =44641045a(15) =143489071a(16) =459165025a(17) =1463588515a(18) =4649045869a(19) =14721978583a(20) =46490458681a(21) =146444944843a(22) =460255540933