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

Terms

a(0) =0a(1) =2a(2) =2a(3) =3a(4) =4a(5) =7a(6) =12a(7) =20a(8) =32a(9) =51a(10) =82a(11) =133a(12) =216a(13) =350a(14) =566a(15) =915a(16) =1480a(17) =2395a(18) =3876a(19) =6272a(20) =10148a(21) =16419a(22) =26566a(23) =42985a(24) =69552a(25) =112538a(26) =182090a(27) =294627a(28) =476716a(29) =771343