|
| |
|
|
A345167
|
|
Numbers k such that the k-th composition in standard order is alternating.
|
|
75
|
|
|
|
0, 1, 2, 4, 5, 6, 8, 9, 12, 13, 16, 17, 18, 20, 22, 24, 25, 32, 33, 34, 38, 40, 41, 44, 45, 48, 49, 50, 54, 64, 65, 66, 68, 70, 72, 76, 77, 80, 81, 82, 88, 89, 96, 97, 98, 102, 108, 109, 128, 129, 130, 132, 134, 140, 141, 144, 145, 148, 152, 153, 160, 161, 162
(list;
graph;
refs;
listen;
history;
text;
internal format)
|
|
|
|
OFFSET
|
1,3
|
|
|
COMMENTS
|
The k-th composition in standard order (graded reverse-lexicographic, A066099) is obtained by taking the set of positions of 1's in the reversed binary expansion of k, prepending 0, taking first differences, and reversing again. This gives a bijective correspondence between nonnegative integers and integer compositions.
A sequence is alternating if it is alternately strictly increasing and strictly decreasing, starting with either. For example, the partition (3,2,2,2,1) has no alternating permutations, even though it does have the anti-run permutations (2,3,2,1,2) and (2,1,2,3,2).
|
|
|
LINKS
|
|
|
|
EXAMPLE
|
The terms together with their binary indices begin:
1: (1) 25: (1,3,1) 66: (5,2)
2: (2) 32: (6) 68: (4,3)
4: (3) 33: (5,1) 70: (4,1,2)
5: (2,1) 34: (4,2) 72: (3,4)
6: (1,2) 38: (3,1,2) 76: (3,1,3)
8: (4) 40: (2,4) 77: (3,1,2,1)
9: (3,1) 41: (2,3,1) 80: (2,5)
12: (1,3) 44: (2,1,3) 81: (2,4,1)
13: (1,2,1) 45: (2,1,2,1) 82: (2,3,2)
16: (5) 48: (1,5) 88: (2,1,4)
17: (4,1) 49: (1,4,1) 89: (2,1,3,1)
18: (3,2) 50: (1,3,2) 96: (1,6)
20: (2,3) 54: (1,2,1,2) 97: (1,5,1)
22: (2,1,2) 64: (7) 98: (1,4,2)
24: (1,4) 65: (6,1) 102: (1,3,1,2)
|
|
|
MATHEMATICA
|
stc[n_]:=Differences[Prepend[Join@@Position[ Reverse[IntegerDigits[n, 2]], 1], 0]]//Reverse;
wigQ[y_]:=Or[Length[y]==0, Length[Split[y]] ==Length[y]&&Length[Split[Sign[Differences[y]]]]==Length[y]-1];
Select[Range[0, 100], wigQ@*stc]
|
|
|
CROSSREFS
|
Partitions with a permutation of this type: A345170, complement A345165.
Factorizations with a permutation of this type: A348379.
A003242 counts anti-run compositions.
A345164 counts alternating permutations of prime indices.
Statistics of standard compositions:
- Number of maximal anti-runs is A333381.
- Number of distinct parts is A334028.
Classes of standard compositions:
- Weakly decreasing compositions (partitions) are A114994.
- Weakly increasing compositions (multisets) are A225620.
- Non-alternating anti-runs are A345169.
Cf. A025048, A025049, A059893, A106356, A238279, A335448, A344604, A344615, A344653, A344742, A345163, A348377.
|
|
|
KEYWORD
|
nonn
|
|
|
AUTHOR
|
|
|
|
STATUS
|
approved
|
| |
|
|
