Number of ways to write n as an ordered sum of 1's, 2's and 3's such that no 2 precedes any 1.

Terms

a(0) =1a(1) =1a(2) =2a(3) =3a(4) =5a(5) =8a(6) =12a(7) =19a(8) =29a(9) =44a(10) =67a(11) =101a(12) =152a(13) =228a(14) =341a(15) =509a(16) =758a(17) =1127a(18) =1673a(19) =2480a(20) =3672a(21) =5431a(22) =8025a(23) =11848a(24) =17479a(25) =25769a(26) =37968a(27) =55912a(28) =82297a(29) =121081