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

Terms

a(0) =-1a(1) =-2a(2) =0a(3) =0a(4) =4a(5) =8a(6) =20a(7) =40a(8) =84a(9) =168a(10) =340a(11) =680a(12) =1364a(13) =2728a(14) =5460a(15) =10920a(16) =21844a(17) =43688a(18) =87380a(19) =174760a(20) =349524a(21) =699048a(22) =1398100a(23) =2796200a(24) =5592404a(25) =11184808a(26) =22369620a(27) =44739240a(28) =89478484a(29) =178956968