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

Terms

a(0) =0a(1) =1a(2) =3a(3) =6a(4) =9a(5) =15a(6) =18a(7) =27a(8) =27a(9) =36a(10) =27a(11) =27a(12) =0a(13) =-27a(14) =-81a(15) =-162a(16) =-243a(17) =-405a(18) =-486a(19) =-729a(20) =-729a(21) =-972a(22) =-729a(23) =-729a(24) =0a(25) =729a(26) =2187a(27) =4374