a(n) = 3*(a(n-2) + 1), with a(0) = 1, a(1) = 3.

Terms

a(0) =1a(1) =3a(2) =6a(3) =12a(4) =21a(5) =39a(6) =66a(7) =120a(8) =201a(9) =363a(10) =606a(11) =1092a(12) =1821a(13) =3279a(14) =5466a(15) =9840a(16) =16401a(17) =29523a(18) =49206a(19) =88572a(20) =147621a(21) =265719a(22) =442866a(23) =797160a(24) =1328601a(25) =2391483a(26) =3985806a(27) =7174452a(28) =11957421a(29) =21523359