Number of compositions of n such that at least two adjacent parts are equal.

Terms

a(0) =0a(1) =0a(2) =1a(3) =1a(4) =4a(5) =9a(6) =18a(7) =41a(8) =89a(9) =185a(10) =388a(11) =810a(12) =1670a(13) =3435a(14) =7040a(15) =14360a(16) =29226a(17) =59347a(18) =120229a(19) =243166a(20) =491086a(21) =990446a(22) =1995410a(23) =4016259a(24) =8076960a(25) =16231746a(26) =32599774a(27) =65437945a(28) =131293192a(29) =263316897