a(n) = a(n-1) + a(n-2) + 1, with a(0)=3, a(1)=12.

Terms

a(0) =3a(1) =12a(2) =16a(3) =29a(4) =46a(5) =76a(6) =123a(7) =200a(8) =324a(9) =525a(10) =850a(11) =1376a(12) =2227a(13) =3604a(14) =5832a(15) =9437a(16) =15270a(17) =24708a(18) =39979a(19) =64688a(20) =104668a(21) =169357a(22) =274026a(23) =443384a(24) =717411a(25) =1160796a(26) =1878208a(27) =3039005a(28) =4917214a(29) =7956220