Number of compositions of n with exactly one part equal to 1 or exactly one part equal to 2.

Terms

a(0) =0a(1) =1a(2) =1a(3) =2a(4) =5a(5) =11a(6) =15a(7) =35a(8) =70a(9) =124a(10) =234a(11) =447a(12) =827a(13) =1529a(14) =2834a(15) =5222a(16) =9587a(17) =17573a(18) =32137a(19) =58641a(20) =106821a(21) =194280a(22) =352824a(23) =639913a(24) =1159238a(25) =2097759a(26) =3792375a(27) =6849778a(28) =12361822a(29) =22292405