a(0)=a(1)=0, a(2)=2; for n >= 3, a(n) = a(n-1) + 4*a(n-3).

Terms

a(0) =0a(1) =0a(2) =2a(3) =2a(4) =2a(5) =10a(6) =18a(7) =26a(8) =66a(9) =138a(10) =242a(11) =506a(12) =1058a(13) =2026a(14) =4050a(15) =8282a(16) =16386a(17) =32586a(18) =65714a(19) =131258a(20) =261602a(21) =524458a(22) =1049490a(23) =2095898a(24) =4193730a(25) =8391690a(26) =16775282a(27) =33550202a(28) =67116962a(29) =134218090