Number of compositions of n into parts 1 and 2 with both parts present.

Terms

a(0) =2a(1) =3a(2) =7a(3) =11a(4) =20a(5) =32a(6) =54a(7) =87a(8) =143a(9) =231a(10) =376a(11) =608a(12) =986a(13) =1595a(14) =2583a(15) =4179a(16) =6764a(17) =10944a(18) =17710a(19) =28655a(20) =46367a(21) =75023a(22) =121392a(23) =196416a(24) =317810a(25) =514227a(26) =832039a(27) =1346267a(28) =2178308a(29) =3524576