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

Terms

a(0) =0a(1) =0a(2) =2a(3) =3a(4) =6a(5) =10a(6) =21a(7) =42a(8) =86a(9) =171a(10) =342a(11) =682a(12) =1365a(13) =2730a(14) =5462a(15) =10923a(16) =21846a(17) =43690a(18) =87381a(19) =174762a(20) =349526a(21) =699051a(22) =1398102a(23) =2796202a(24) =5592405a(25) =11184810a(26) =22369622a(27) =44739243a(28) =89478486a(29) =178956970