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

Terms

a(0) =1a(1) =2a(2) =3a(3) =5a(4) =9a(5) =15a(6) =25a(7) =43a(8) =73a(9) =123a(10) =209a(11) =355a(12) =601a(13) =1019a(14) =1729a(15) =2931a(16) =4969a(17) =8427a(18) =14289a(19) =24227a(20) =41081a(21) =69659a(22) =118113a(23) =200275a(24) =339593a(25) =575819a(26) =976369a(27) =1655555a(28) =2807193a(29) =4759931