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By Bart De Bruyn

This publication supplies an creation to the sector of occurrence Geometry via discussing the elemental households of point-line geometries and introducing the various mathematical options which are crucial for his or her research. The households of geometries coated during this publication contain between others the generalized polygons, close to polygons, polar areas, twin polar areas and designs. additionally a number of the relationships among those geometries are investigated. Ovals and ovoids of projective areas are studied and a few functions to specific geometries can be given. A separate bankruptcy introduces the mandatory mathematical instruments and methods from graph concept. This bankruptcy itself will be considered as a self-contained creation to strongly average and distance-regular graphs.

This ebook is largely self-contained, in basic terms assuming the data of simple notions from (linear) algebra and projective and affine geometry. just about all theorems are observed with proofs and a listing of routines with complete suggestions is given on the finish of the publication. This booklet is geared toward graduate scholars and researchers within the fields of combinatorics and occurrence geometry.

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Since they are all contained in the subspace V whose dimension is Mi (M2 i +3) , we necessarily have v ≤ Mi (M2 i +3) . 9 are called the absolute bounds. 10 A connected regular graph is strongly regular if and only if its adjacency matrix has exactly three distinct eigenvalues. Proof. Suppose Γ is a connected strongly regular graph with parameters (v, k, λ, μ). 2. 6 implies that Γ has three distinct eigenvalues. Conversely, suppose that Γ is a connected regular graph of valency k having three distinct eigenvalues k, R1 and R2 .

By definition, the point-line dual of a generalized polygon is again a generalized polygon. Ordinary n-gons with n ≥ 3 are examples of generalized n-gons. The generalized 3-gons are precisely the possibly degenerate projective planes. We will prove the following facts in Chapter 5: • a generalized n-gon, n ≥ 2, has diameter n 2 ; • every generalized n-gon, n ≥ 3, is a near n-gon; • the double of a generalized n-gon, n ≥ 2, is a generalized 2n-gon. In the following two theorems, we collect some restrictions that must be satisfied by orders of finite generalized polygons.

Dually, if α = t + 1, then the partial geometry is the point-line dual of a linear space. The partial geometries with parameters (s, t, s) are precisely the dual nets of order (s, t) and the partial geometries with parameters (s, t, t) are precisely the nets of order (s, t). The collinearity graph of a partial geometry with parameters (s, t, α), α = s + 1, is a strongly regular graph with parameters (v, k, λ, μ) = (s + 1)(st + α) , s(t + 1), s − 1 + t(α − 1), α(t + 1) . α Since v ∈ N, α must be a divisor of (s + 1)st.

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