a(n) = a(n-1)+a(n-2)+a(n-3)+2*a(n-4), a(0)=1, a(1)=3, a(2)=7, a(3)=15.

Terms

a(0) =1a(1) =3a(2) =7a(3) =15a(4) =27a(5) =55a(6) =111a(7) =223a(8) =443a(9) =887a(10) =1775a(11) =3551a(12) =7099a(13) =14199a(14) =28399a(15) =56799a(16) =113595a(17) =227191a(18) =454383a(19) =908767a(20) =1817531a(21) =3635063a(22) =7270127a(23) =14540255a(24) =29080507a(25) =58161015a(26) =116322031a(27) =232644063a(28) =465288123a(29) =930576247