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

Terms

a(0) =0a(1) =2a(2) =3a(3) =6a(4) =10a(5) =17a(6) =28a(7) =46a(8) =75a(9) =122a(10) =198a(11) =321a(12) =520a(13) =842a(14) =1363a(15) =2206a(16) =3570a(17) =5777a(18) =9348a(19) =15126a(20) =24475a(21) =39602a(22) =64078a(23) =103681a(24) =167760a(25) =271442a(26) =439203a(27) =710646a(28) =1149850a(29) =1860497