a(n) = (1/6)*(3^n + (-2)^n - 1).

Terms

a(0) =0a(1) =2a(2) =3a(3) =16a(4) =35a(5) =132a(6) =343a(7) =1136a(8) =3195a(9) =10012a(10) =29183a(11) =89256a(12) =264355a(13) =799892a(14) =2386023a(15) =7185376a(16) =21501515a(17) =64613772a(18) =193622863a(19) =581305496a(20) =1743042675a(21) =5230875652a(22) =15689131703a(23) =47074385616a(24) =141209175835a(25) =423655489532