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

Terms

a(0) =1a(1) =1a(2) =2a(3) =7a(4) =26a(5) =91a(6) =302a(7) =967a(8) =3026a(9) =9331a(10) =28502a(11) =86527a(12) =261626a(13) =788971a(14) =2375102a(15) =7141687a(16) =21457826a(17) =64439011a(18) =193448102a(19) =580606447a(20) =1742343626a(21) =5228079451a(22) =15686335502a(23) =47063200807a(24) =141197991026a(25) =423610750291