A129838 Number of up/down (or down/up) compositions of n into distinct parts.

1, 1, 1, 2, 2, 3, 5, 6, 8, 11, 18, 21, 30, 38, 52, 78, 97, 128, 170, 222, 285, 421, 510, 683, 872, 1148, 1440, 1893, 2576, 3209, 4151, 5313, 6784, 8615, 10969, 13755, 18573, 22713, 29173, 36536, 46705, 57899, 73696, 91076, 114777, 148531, 182813, 228938, 287042

Offset: 0

Author

Vladeta Jovovic, May 21 2007

Keywords

• easy
• nonn

Comments

Original name was: Number of alternating compositions of n into distinct parts.

A composition is up/down if it is alternately strictly increasing and strictly decreasing, starting with an increase. - Gus Wiseman, Jan 15 2022

Examples

			From _Gus Wiseman_, Jan 15 2022: (Start)
The a(1) = 1 through a(8) = 8 up/down strict compositions (non-strict A025048):
  (1)  (2)  (3)    (4)    (5)    (6)      (7)      (8)
            (1,2)  (1,3)  (1,4)  (1,5)    (1,6)    (1,7)
                          (2,3)  (2,4)    (2,5)    (2,6)
                                 (1,3,2)  (3,4)    (3,5)
                                 (2,3,1)  (1,4,2)  (1,4,3)
                                          (2,4,1)  (1,5,2)
                                                   (2,5,1)
                                                   (3,4,1)
The a(1) = 1 through a(8) = 8 down/up strict compositions (non-strict A025049):
  (1)  (2)  (3)    (4)    (5)    (6)      (7)      (8)
            (2,1)  (3,1)  (3,2)  (4,2)    (4,3)    (5,3)
                          (4,1)  (5,1)    (5,2)    (6,2)
                                 (2,1,3)  (6,1)    (7,1)
                                 (3,1,2)  (2,1,4)  (2,1,5)
                                          (4,1,2)  (3,1,4)
                                                   (4,1,3)
                                                   (5,1,2)
(End)
		

Links

• Alois P. Heinz, Table of n, a(n) for n = 0..10000

Crossrefs

The case of permutations is A000111.

This is the up/down case of A032020.

This is the strict case of A129852/A129853, strong A025048/A025049.

The undirected version is A349054.

A001250 = alternating permutations, complement A348615.

A003242 = Carlitz compositions, complement A261983.

A011782 = compositions, unordered A000041.

A025047 = alternating compositions, complement A345192.

A349052 = weakly alternating compositions, complement A349053.

Cf. A003056, A008289, A008965, A015723, A072706, A128761, A218074, A345165, A345170, A345195, A349800.

Programs

Maple

g:= proc(u, o) option remember;
      `if`(u+o=0, 1, add(g(o-1+j, u-j), j=1..u))
    end:
b:= proc(n, k) option remember; `if`(k<0 or n<0, 0,
      `if`(k=0, `if`(n=0, 1, 0), b(n-k, k)+b(n-k, k-1)))
    end:
a:= n-> add(b(n, k)*g(k, 0), k=0..floor((sqrt(8*n+1)-1)/2)):
seq(a(n), n=0..60);  # Alois P. Heinz, Dec 22 2021

Mathematica

whkQ[y_]:=And@@Table[If[EvenQ[m],y[[m]]y[[m+1]]],{m,1,Length[y]-1}];
Table[Length[Select[Join@@Permutations/@ Select[IntegerPartitions[n],UnsameQ@@#&],whkQ]],{n,0,15}] (* Gus Wiseman, Jan 15 2022 *)

Formula

G.f.: Sum_{k>=0} A000111(k)*x^(k*(k+1)/2)/Product_{i=1..k} (1-x^i). - Vladeta Jovovic, May 24 2007

a(n) = Sum_{k=0..A003056(n)} A000111(k) * A008289(n,k). - Alois P. Heinz, Dec 22 2021

a(n) = (A349054(n) + 1)/2. - Gus Wiseman, Jan 15 2022

Extensions

a(0)=1 prepended by Alois P. Heinz, Dec 22 2021

Name changed from "alternating" to "up/down" by Gus Wiseman, Jan 15 2022