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

Terms

a(0) =0a(1) =0a(2) =1a(3) =1a(4) =3a(5) =4a(6) =9a(7) =14a(8) =28a(9) =47a(10) =89a(11) =155a(12) =286a(13) =507a(14) =924a(15) =1652a(16) =2993a(17) =5373a(18) =9707a(19) =17460a(20) =31501a(21) =56714a(22) =102256a(23) =184183a(24) =331981a(25) =598091a(26) =1077870a(27) =1942071a(28) =3499720a(29) =6305992