By Bart De Bruyn

This booklet provides an creation to the sphere of occurrence Geometry by way of discussing the fundamental households of point-line geometries and introducing a number of the mathematical recommendations which are crucial for his or her research. The households of geometries coated during this booklet contain between others the generalized polygons, close to polygons, polar areas, twin polar areas and designs. additionally some of the relationships among those geometries are investigated. Ovals and ovoids of projective areas are studied and a few functions to specific geometries could be given. A separate bankruptcy introduces the mandatory mathematical instruments and methods from graph idea. This bankruptcy itself will be considered as a self-contained advent to strongly general and distance-regular graphs.

This publication is largely self-contained, basically assuming the information of simple notions from (linear) algebra and projective and affine geometry. just about all theorems are observed with proofs and an inventory of workouts with complete options is given on the finish of the e-book. This e-book is geared toward graduate scholars and researchers within the fields of combinatorics and prevalence geometry.

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Derived from a different consultation on Low Dimensional Topology equipped and performed by way of Dr Lomonaco on the American Mathematical Society assembly held in San Francisco, California, January 7-11, 1981

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**Example text**

0 0 0 ··· 0 X Hence, det(X · I − J) = (X − v)X v−1 . So, the eigenvalues of the matrix J are v (with multiplicity 1) and 0 (with multiplicity v − 1). 6 (1) We have A2 = k · I + λ · A + μ · (J − I − A). (2) Suppose μ = 0. So, Γ is the disjoint union of r ≥ 2 complete graphs on v m ≥ 2 vertices, where r = k+1 and m = k + 1. Then Γ has two distinct eigenvalues k and −1 with respective multiplicities r and v − r. (3) If μ = 0, then Γ has three distinct eigenvalues k, R1 and R2 , where k > R1 ≥ 0 and R2 < −1.

The set M of all generators of Q+ (2n − 1, F) can be partitioned into two subsets M+ and M− such that two generators belong to the same subset M for some ∈ {+, −} if and only if they intersect in a subspace of even co-dimension. The sets M+ and M− are called the two families of generators of Q+ (2n − 1, F). For every ∈ {+, −}, let S be the following point-line geometry: • the points of S are the elements of M ; • the lines of S are the (n−3)-dimensional singular subspaces of Q+ (2n− 1, F); • incidence is reverse containment.

Note that the condition that each equivalence class contains at least two points is necessary to exclude some geometries that do not arise as duals of nets (like lines and K1,a ’s with a ≥ 2). 2 In fact, this construction would also produce a net if the original geometry A would be an arbitrary net (and not merely an aﬃne plane). 6 - Designs If S = (P, L, I) is an aﬃne plane and L is a parallel class of lines of S, then S = (P, L \ L , I ), where I is the restriction of I to P × (L \ L ) is a net and also a dual net.