a(n) = (n/4)*3^(n/2)*((1+sqrt(3))^2+(-1)^n*(1-sqrt(3))^2).

Terms

a(0) =0a(1) =3a(2) =12a(3) =27a(4) =72a(5) =135a(6) =324a(7) =567a(8) =1296a(9) =2187a(10) =4860a(11) =8019a(12) =17496a(13) =28431a(14) =61236a(15) =98415a(16) =209952a(17) =334611a(18) =708588a(19) =1121931a(20) =2361960a(21) =3720087a(22) =7794468a(23) =12223143a(24) =25509168a(25) =39858075a(26) =82904796a(27) =129140163a(28) =267846264a(29) =416118303