A335449 Number of (1,2,1)-avoiding permutations of the prime indices of n.

1, 1, 1, 1, 1, 2, 1, 1, 1, 2, 1, 2, 1, 2, 2, 1, 1, 3, 1, 2, 2, 2, 1, 2, 1, 2, 1, 2, 1, 6, 1, 1, 2, 2, 2, 3, 1, 2, 2, 2, 1, 6, 1, 2, 2, 2, 1, 2, 1, 3, 2, 2, 1, 4, 2, 2, 2, 2, 1, 6, 1, 2, 2, 1, 2, 6, 1, 2, 2, 6, 1, 3, 1, 2, 3, 2, 2, 6, 1, 2, 1, 2, 1, 6, 2, 2, 2

Offset

1,6

Comments

Depends only on unsorted prime signature (A124010), but not only on sorted prime signature (A118914).

A prime index of n is a number m such that prime(m) divides n. The multiset of prime indices of n is row n of A112798.

We define a pattern to be a finite sequence covering an initial interval of positive integers. Patterns are counted by A000670. 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).

Links

Table of n, a(n) for n=1..87.

Wikipedia, Permutation pattern

Gus Wiseman, Sequences counting and ranking compositions by the patterns they match or avoid.

Example

The a(n) permutations for n = 2, 10, 36, 54, 324, 30, 1458, 90:
  (1)  (13)  (1122)  (1222)  (112222)  (123)  (1222222)  (1223)
       (31)  (2112)  (2122)  (211222)  (132)  (2122222)  (1322)
             (2211)  (2212)  (221122)  (213)  (2212222)  (2123)
                     (2221)  (222112)  (231)  (2221222)  (2213)
                             (222211)  (312)  (2222122)  (2231)
                                       (321)  (2222212)  (3122)
                                              (2222221)  (3212)
                                                         (3221)

Mathematica

primeMS[n_]:=If[n==1, {}, Flatten[Cases[FactorInteger[n], {p_, k_}:>Table[PrimePi[p], {k}]]]];
Table[Length[Select[Permutations[primeMS[n]], !MatchQ[#, {___, x_, ___, y_, ___, x_, ___}/; x<y]&]], {n, 100}]

Crossrefs

The matching version is A335446.

Patterns are counted by A000670.

(1,2,1)-avoiding patterns are counted by A001710.

Permutations of prime indices are counted by A008480.

Unsorted prime signature is A124010. Sorted prime signature is A118914.

(1,2,1) and (2,1,2)-avoiding permutations of prime indices are counted by A333175.

STC-numbers of permutations of prime indices are A333221.

(1,2,1) and (2,1,2)-avoiding permutations of prime indices are A335448.

Patterns matched by standard compositions are counted by A335454.

(1,2,1) or (2,1,2)-matching permutations of prime indices are A335460.

(1,2,1) and (2,1,2)-matching permutations of prime indices are A335462.

Dimensions of downsets of standard compositions are A335465.

(1,2,1)-avoiding compositions are ranked by A335467.

(1,2,1)-avoiding compositions are counted by A335471.

Cf. A056239, A056986, A112798, A158005, A181796, A335452, A335463.

Sequence in context: A305150 A351202 A333145 * A318464 A051265 A008647

Adjacent sequences: A335446 A335447 A335448 * A335450 A335451 A335452

Keyword

nonn

Author

Gus Wiseman, Jun 14 2020

Status

approved