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

Terms

a(0) =-1a(1) =0a(2) =0a(3) =2a(4) =4a(5) =10a(6) =20a(7) =42a(8) =84a(9) =170a(10) =340a(11) =682a(12) =1364a(13) =2730a(14) =5460a(15) =10922a(16) =21844a(17) =43690a(18) =87380a(19) =174762a(20) =349524a(21) =699050a(22) =1398100a(23) =2796202a(24) =5592404a(25) =11184810a(26) =22369620a(27) =44739242a(28) =89478484a(29) =178956970