a(n) = 2*a(n-1) + 2*a(n-3) + a(n-4) - 1, with a(0) = 0 = a(1), a(2) = 2, and a(3) = 3.

Terms

a(0) =0a(1) =0a(2) =2a(3) =3a(4) =5a(5) =13a(6) =33a(7) =78a(8) =186a(9) =450a(10) =1088a(11) =2625a(12) =6335a(13) =15295a(14) =36927a(15) =89148a(16) =215220a(17) =519588a(18) =1254398a(19) =3028383a(20) =7311161a(21) =17650705a(22) =42612573a(23) =102875850a(24) =248364270a(25) =599604390a(26) =1447573052a(27) =3494750493a(28) =8437074035a(29) =20368898563