cp's OEIS Frontend

Number of n x n binary matrices without zero rows and with distinct rows up to permutation of rows, cf. A014070.

Row 0 of square array A136555.

From Gus Wiseman, Dec 19 2023: (Start)

Also the number of n-element sets of nonempty subsets of {1..n}, or set-systems with n vertices and n edges (not necessarily covering). The covering case is A054780. For example, the a(3) = 35 set-systems are:

{1}{2}{3} {1}{2}{12} {1}{2}{123} {1}{12}{123} {12}{13}{123}

{1}{2}{13} {1}{3}{123} {1}{13}{123} {12}{23}{123}

{1}{2}{23} {1}{12}{13} {1}{23}{123} {13}{23}{123}

{1}{3}{12} {1}{12}{23} {2}{12}{123}

{1}{3}{13} {1}{13}{23} {2}{13}{123}

{1}{3}{23} {2}{3}{123} {2}{23}{123}

{2}{3}{12} {2}{12}{13} {3}{12}{123}

{2}{3}{13} {2}{12}{23} {3}{13}{123}

{2}{3}{23} {2}{13}{23} {3}{23}{123}

{3}{12}{13} {12}{13}{23}

{3}{12}{23}

{3}{13}{23}

Of these, only {{1},{2},{1,2}}, {{1},{3},{1,3}}, and {{2},{3},{2,3}} do not cover the vertex set.

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In a multiset partition, two vertices are equivalent if in every block the multiplicity of the first is equal to the multiplicity of the second. The T_0 condition means that there are no equivalent vertices.

The weight of a multiset partition is the sum of sizes of its parts. Weight is generally not the same as number of vertices.

The axiom of choice says that, given any set of nonempty sets Y, it is possible to choose a set containing an element from each. The strict version requires this set to have the same cardinality as Y, meaning no element is chosen more than once.

Includes all set-systems with more edges than covered vertices, but this condition is not sufficient.

The axiom of choice says that, given any set of nonempty sets Y, it is possible to choose a set containing an element from each. The strict version requires this set to have the same cardinality as Y, meaning no element is chosen more than once.

Excludes all set-systems with more edges than covered vertices, but this condition is not sufficient.

A cover of a set is a T_0-cover if for every two distinct points of the set there exists a member (block) of the cover containing one but not the other point.

Also, T(n,m) is the number of n X m (0,1)-matrices with pairwise distinct nonzero columns and pairwise distinct nonzero rows, up to permutation of columns.

In a multiset partition, two vertices are equivalent if in every block the multiplicity of the first is equal to the multiplicity of the second. The T_0 condition means that there are no equivalent vertices.

The weight of a multiset partition is the sum of sizes of its parts. Weight is generally not the same as number of vertices.

The dual of a multiset partition has, for each vertex, one block consisting of the indices (or positions) of the blocks containing that vertex. For example, the dual of {{1,2},{2,3}} is {{1},{1,2},{2}}. The T_0 condition means that the dual is strict (no repeated edges).

Same as A059523 except with a(1) = 1 instead of 2.

Alternatively, these are set-systems covering n vertices whose dual is a (strict) antichain. A set-system is a finite set of finite nonempty sets. The dual of a set-system has, for each vertex, one edge consisting of the indices (or positions) of the edges containing that vertex. An antichain is a set of sets, none of which is a subset of any other.

A set-system is a finite set of finite nonempty sets. The dual of a set-system has, for each vertex, one edge consisting of the indices (or positions) of the edges containing that vertex. For example, the dual of {{1,2},{2,3}} is {{1},{1,2},{2}}. The T_1 condition means that the dual is a (strict) antichain, meaning that none of its edges is a subset of any other.

A binary index of n is any position of a 1 in its reversed binary expansion. The binary indices of n are row n of A048793. We define the set-system with BII-number n to be obtained by taking the binary indices of each binary index of n. Every finite set of finite nonempty sets has a different BII-number. For example, 18 has reversed binary expansion (0,1,0,0,1), and since the binary indices of 2 and 5 are {2} and {1,3} respectively, the BII-number of {{2},{1,3}} is 18. Elements of a set-system are sometimes called edges.

From Gus Wiseman, Jan 02 2024: (Start)

Also the number of n-element sets of finite nonempty subsets of {1..n}, including a unique singleton, such that there is exactly one way to choose a different element from each. For example, the a(0) = 0 through a(3) = 15 set-systems are:

. {{1}} {{1},{1,2}} {{1},{1,2},{1,3}}

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

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

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

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

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

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

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

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

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

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

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

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

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

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

These set-systems are all connected.

The case of labeled graphs is A000169.

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