a(n) = a(n-1) + a(n-2) + 1, with a(0)=3, a(1)=9.

Terms

a(0) =3a(1) =9a(2) =13a(3) =23a(4) =37a(5) =61a(6) =99a(7) =161a(8) =261a(9) =423a(10) =685a(11) =1109a(12) =1795a(13) =2905a(14) =4701a(15) =7607a(16) =12309a(17) =19917a(18) =32227a(19) =52145a(20) =84373a(21) =136519a(22) =220893a(23) =357413a(24) =578307a(25) =935721a(26) =1514029a(27) =2449751a(28) =3963781a(29) =6413533