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A335477
Numbers k such that the k-th composition in standard order (A066099) matches the pattern (2,2,1).
3
21, 43, 45, 53, 73, 85, 86, 87, 91, 93, 107, 109, 117, 146, 147, 149, 153, 165, 169, 171, 172, 173, 174, 175, 181, 182, 183, 187, 189, 201, 213, 214, 215, 219, 221, 235, 237, 245, 273, 277, 293, 294, 295, 297, 299, 301, 306, 307, 309, 313, 325, 329, 331, 333
OFFSET
1,1
COMMENTS
A composition of n is a finite sequence of positive integers summing to n. 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.
We define a pattern to be a finite sequence covering an initial interval of positive integers. Patterns are counted by A000670 and ranked by A333217. A sequence S is said to match a pattern P if there is a not necessarily contiguous subsequence of S whose parts have the same relative order as P. For example, (3,1,1,3) matches (1,1,2), (2,1,1), and (2,1,2), but avoids (1,2,1), (1,2,2), and (2,2,1).
EXAMPLE
The sequence of terms together with the corresponding compositions begins:
21: (2,2,1)
43: (2,2,1,1)
45: (2,1,2,1)
53: (1,2,2,1)
73: (3,3,1)
85: (2,2,2,1)
86: (2,2,1,2)
87: (2,2,1,1,1)
91: (2,1,2,1,1)
93: (2,1,1,2,1)
107: (1,2,2,1,1)
109: (1,2,1,2,1)
117: (1,1,2,2,1)
146: (3,3,2)
147: (3,3,1,1)
MATHEMATICA
stc[n_]:=Reverse[Differences[Prepend[Join@@Position[Reverse[IntegerDigits[n, 2]], 1], 0]]];
Select[Range[0, 100], MatchQ[stc[#], {___, x_, ___, x_, ___, y_, ___}/; x>y]&]
CROSSREFS
The complement A335524 is the avoiding version.
The (1,2,2)-matching version is A335475.
Patterns matching this pattern are counted by A335509 (by length).
Permutations of prime indices matching this pattern are counted by A335453.
These compositions are counted by A335472 (by sum).
Constant patterns are counted by A000005 and ranked by A272919.
Permutations are counted by A000142 and ranked by A333218.
Patterns are counted by A000670 and ranked by A333217.
Non-unimodal compositions are counted by A115981 and ranked by A335373.
Combinatory separations are counted by A269134.
Patterns matched by standard compositions are counted by A335454.
Minimal patterns avoided by a standard composition are counted by A335465.
Sequence in context: A039344 A043167 A043947 * A263877 A041870 A041868
KEYWORD
nonn
AUTHOR
Gus Wiseman, Jun 18 2020
STATUS
approved