Partial linear space

A partial linear space (also semilinear or near-linear space) is a basic incidence structure in the field of incidence geometry, that carries slightly less structure than a linear space. The notion is equivalent to that of a linear hypergraph.

Definition

Let an incidence structure, for which the elements of are called points and the elements of are called lines. S is a partial linear space, if the following axioms hold:

  • any line is incident with at least two points
  • any pair of distinct points is incident with at most one line

If there is a unique line incident with every pair of distinct points, then we get a linear space.

Properties

The De Bruijn–Erdős theorem (incidence geometry) shows that in any finite linear space which is not a single point or a single line, we have .

Examples

gollark: I do free apartments.
gollark: Not really.
gollark: Wait, *4*00KST?
gollark: 100KST per account seems reasonable.
gollark: I don't think the rules should work that way, honestly.

References

  • Shult, Ernest E. (2011), Points and Lines, Universitext, Springer, doi:10.1007/978-3-642-15627-4, ISBN 978-3-642-15626-7.
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