a(n) = (4*3^n - 5*2^n + (-2)^n)/20.

Terms

a(0) =0a(1) =0a(2) =1a(3) =3a(4) =13a(5) =39a(6) =133a(7) =399a(8) =1261a(9) =3783a(10) =11605a(11) =34815a(12) =105469a(13) =316407a(14) =953317a(15) =2859951a(16) =8596237a(17) =25788711a(18) =77431669a(19) =232295007a(20) =697147165a(21) =2091441495a(22) =6275373061a(23) =18826119183a(24) =56482551853a(25) =169447655559a(26) =508359743893