cp's OEIS Frontend

A balanced reduced multisystem is either a finite multiset, or a multiset partition with at least two parts, not all of which are singletons, of a balanced reduced multisystem. The weight of an atom is 1, while the weight of a multiset is the sum of weights of its elements.

First differs from A045778 at a(24) = 4, A045778(24) = 5.

Also the number of multisets that can be obtained by taking the sums of prime indices of each factor in a factorization of n into distinct factors > 1.

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.

A multiset partition can be regarded as an arrow in the poset of integer partitions. For example, we have {{1},{1,2},{1,3},{1,2,3}}: {1,1,1,1,2,2,3,3} -> {1,3,4,6}, or (33221111) -> (6431) (depending on notation).

Sets of multisets are generally not transitive. For example, we have arrows: {{1},{2},{1,2}}: {1,1,2,2} -> {1,2,3} and {{1,2},{3}}: {1,2,3} -> {3,3}, but there is no set of multisets {1,1,2,2} -> {3,3}.

A balanced reduced multisystem is either a finite multiset, or a multiset partition with at least two parts, not all of which are singletons, of a balanced reduced multisystem.

This poset is equivalent to the poset of multiset partitions of the prime indices of n, ordered by refinement.

Number of proper (n-1)-times partitions of n, cf. A327639.

Might be called "Half-Factorial numbers" analog to the "Half-Catalan numbers" (A000992).

The recursion formula is a special case of the formula given in A327729.

a(n+1)/(n*a(n)) tends to 0.67617164... - Vaclav Kotesovec, Apr 28 2020

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. A multiset whose multiplicities are the prime indices of n (such as row n of A305936) is generally not the same as the multiset of prime indices of n. For example, the prime indices of 12 are {1,1,2}, while a multiset whose multiplicities are {1,1,2} is {1,1,2,3}.

A balanced reduced multisystem is either a finite multiset, or a multiset partition with at least two parts, not all of which are singletons, of a balanced reduced multisystem.

a(n) is the number of integer partitions finer than (n, ..., 3, 2, 1) in the poset of integer partitions of 1 + 2 + ... + n ordered by refinement.

a(n+1)/a(n) appears to converge as n -> oo. - Chai Wah Wu, Nov 14 2018

The Heinz number of an integer partition (y_1, ..., y_k) is prime(y_1) * ... * prime(y_k).

These partitions are those that are finer than (k, ..., 3, 2, 1) in the poset of integer partitions of 1 + 2 + ... + k, for some k, ordered by refinement.

The triangle reflects the Jordan-decomposition of the matrix of Stirling numbers of the second kind. A display of the matrix formula can be found at the Helms link which also explains the generation rule for the A()-numbers in a different way. - Gottfried Helms Apr 19 2014

From Gus Wiseman, Jan 02 2020: (Start)

Also the number of balanced reduced multisystems with atoms {1..n} and depth k. A balanced reduced multisystem is either a finite multiset, or a multiset partition with at least two parts, not all of which are singletons, of a balanced reduced multisystem. For example, row n = 4 counts the following multisystems:

{1,2,3,4} {{1},{2,3,4}} {{{1}},{{2},{3,4}}}

{{1,2},{3,4}} {{{1},{2}},{{3,4}}}

{{1,2,3},{4}} {{{1},{2,3}},{{4}}}

{{1,2,4},{3}} {{{1,2}},{{3},{4}}}

{{1,3},{2,4}} {{{1,2},{3}},{{4}}}

{{1,3,4},{2}} {{{1},{2,4}},{{3}}}

{{1,4},{2,3}} {{{1,2},{4}},{{3}}}

{{1},{2},{3,4}} {{{1}},{{3},{2,4}}}

{{1},{2,3},{4}} {{{1},{3}},{{2,4}}}

{{1,2},{3},{4}} {{{1,3}},{{2},{4}}}

{{1},{2,4},{3}} {{{1,3},{2}},{{4}}}

{{1,3},{2},{4}} {{{1},{3,4}},{{2}}}

{{1,4},{2},{3}} {{{1,3},{4}},{{2}}}

{{{1}},{{4},{2,3}}}

{{{1},{4}},{{2,3}}}

{{{1,4}},{{2},{3}}}

{{{1,4},{2}},{{3}}}

{{{1,4},{3}},{{2}}}

(End)

From Harry Richman, Mar 30 2023: (Start)

Equivalently, T(n,k) is the number of length-k chains from minimum to maximum in the lattice of set partitions of {1..n} ordered by refinement. For example, row n = 4 counts the following chains, leaving out the minimum {1|2|3|4} and maximum {1234}:

(empty) {12|3|4} {12|3|4} < {123|4}

{13|2|4} {12|3|4} < {124|3}

{14|2|3} {12|3|4} < {12|34}

{1|23|4} {13|2|4} < {123|4}

{1|24|3} {13|2|4} < {134|2}

{1|2|34} {13|2|4} < {13|24}

{123|4} {14|2|3} < {124|3}

{124|3} {14|2|3} < {134|2}

{134|2} {14|2|3} < {14|23}

{1|234} {1|23|4} < {123|4}

{12|34} {1|23|4} < {1|234}

{13|24} {1|23|4} < {14|23}

{14|23} {1|24|3} < {124|3}

{1|24|3} < {1|234}

{1|24|3} < {13|24}

{1|2|34} < {134|2}

{1|2|34} < {1|234}

{1|2|34} < {12|34}

(End)

Also the number of cells of dimension k in the fine subdivision of the Bergman complex of the complete graph on n vertices. - Harry Richman, Mar 30 2023