Number of compositions of n+2 having exactly two parts equal to 1.

Terms

a(0) =1a(1) =0a(2) =3a(3) =3a(4) =9a(5) =15a(6) =31a(7) =57a(8) =108a(9) =199a(10) =366a(11) =666a(12) =1205a(13) =2166a(14) =3873a(15) =6891a(16) =12207a(17) =21537a(18) =37859a(19) =66327a(20) =115842a(21) =201743a(22) =350412a(23) =607140a(24) =1049545a(25) =1810428a(26) =3116655a(27) =5355219a(28) =9185349a(29) =15728547