Number of compositions of n matching the pattern (1,2,1).

Terms

a(0) =0a(1) =0a(2) =0a(3) =0a(4) =1a(5) =3a(6) =9a(7) =24a(8) =61a(9) =141a(10) =322a(11) =713a(12) =1543a(13) =3289a(14) =6907a(15) =14353a(16) =29604a(17) =60640a(18) =123522a(19) =250645a(20) =506808a(21) =1022197a(22) =2057594a(23) =4135358a(24) =8301139a(25) =16648165a(26) =33364948a(27) =66831721a(28) =133814251a(29) =267850803