Incidence structure

Definition
An incidence structure is an incidence system with two types: points and blocks.

Equivalently, it can be viewed as a bipartite graph with the two parts separated, marked, and labelled as the points versus the blocks.

Dual incidence structures
The dual of an incidence structure is the incidence structure obtained by interchanging the role of points and blocks. In graph-theoretic terms, it is obtained by relabelling the vertex set of points as blocks, and the vertex set of blocks as points.

Complementary incidence structure
The complementary incidence structure to a given incidence structure is the incidence structure obtained by taking the complement of the graph associated with the incidence structure.

Parameters
Let $$v$$ be the number of points and $$b$$ the number of blocks. We define $$v_m$$ as the average number of points on $$m$$ blocks and $$b_m$$ denotes the average number of blocks incident on $$m$$ points.