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

Terms

a(0) =1a(1) =2a(2) =4a(3) =9a(4) =20a(5) =44a(6) =97a(7) =214a(8) =472a(9) =1041a(10) =2296a(11) =5064a(12) =11169a(13) =24634a(14) =54332a(15) =119833a(16) =264300a(17) =582932a(18) =1285697a(19) =2835694a(20) =6254320a(21) =13794337a(22) =30424368a(23) =67103056a(24) =148000449a(25) =326425266a(26) =719953588a(27) =1587907625a(28) =3502240516a(29) =7724434620