a(n) = 3*a(n-2) - a(n-4), a(0)=0, a(1)=1, a(2)=5, a(3)=4.

Terms

a(0) =0a(1) =1a(2) =5a(3) =4a(4) =15a(5) =11a(6) =40a(7) =29a(8) =105a(9) =76a(10) =275a(11) =199a(12) =720a(13) =521a(14) =1885a(15) =1364a(16) =4935a(17) =3571a(18) =12920a(19) =9349a(20) =33825a(21) =24476a(22) =88555a(23) =64079a(24) =231840a(25) =167761a(26) =606965a(27) =439204a(28) =1589055a(29) =1149851