a(n) = (1/9)(2^(n+3)-(-1)^n(3n-1)).

Terms

a(0) =1a(1) =2a(2) =3a(3) =8a(4) =13a(5) =30a(6) =55a(7) =116a(8) =225a(9) =458a(10) =907a(11) =1824a(12) =3637a(13) =7286a(14) =14559a(15) =29132a(16) =58249a(17) =116514a(18) =233011a(19) =466040a(20) =932061a(21) =1864142a(22) =3728263a(23) =7456548a(24) =14913073a(25) =29826170a(26) =59652315a(27) =119304656a(28) =238609285a(29) =477218598