A386599 Number of words covering the alphabet [n] that are irreducible with respect to C, where C is a subset of the alphabet such that all letters in C appear in weakly increasing order within the word.

1, 1, 2, 7, 34, 240, 2613, 46488, 1335175, 57040060

Offset

0,3

Comments

Here a word is irreducible if no letters can be removed without changing the maximal cardinality of C. The lengths of these words range from n to (n-1)*2.

Links

Table of n, a(n) for n=0..9.

Example

a(3) = 7 counts: (2,3,1,2), (3,2,1), (1,3,2), (2,1,3), (2,3,1), (3,1,2), (1,2,3).

Crossrefs

Row sums of A390670.

Cf. A386891, A387336, A390275.

Sequence in context: A294466 A029894 A110313 * A000944 A136310 A222868

Adjacent sequences: A386596 A386597 A386598 * A386600 A386601 A386602

Keyword

nonn,more

Author

John Tyler Rascoe, Nov 28 2025

Extensions

a(9) from Sean A. Irvine, Dec 03 2025

Status

approved