and

and define a matrix A by

This matrix A is the so called incidence matrix of the incidence structure
.
Of course, the incidence matrix depends on the enumerations of points
and lines chosen. When using the same enumerations as above, the
incidence matrix of the dual incidence
structure is the transpose A^{t} of A.

The matrix AA^{t}
is the adjacency matrix, its entry at an off diagonal position (i,j)
is the number of lines connecting the two points
and
.

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