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Strongly regular graphs from differences of quadrics

Authors: Nicholas A. Hamilton;

Strongly regular graphs from differences of quadrics

Abstract

Suppose in the finite projective space \(\text{PG}(n,q)\) there exists a set \(\mathcal K\) of points such that every hyperplane of \(\text{PG}(n,q)\) contains either \(h\) or \(h'\) points of \(\mathcal K\). It is well known that a strongly regular graph \(\Gamma\) may be obtained from \(\mathcal K\). Embed \(\text{PG}(n,q)\) in \(\text{PG}(n+1,q)\). The vertices of \(\Gamma\) are the points of \(\text{PG}(n+1,q)- \text{PG}(n,q)\), and two vertices are adjacent if and only if the line joining them meets \(\text{PG}(n,q)\) in a point of \(\mathcal K\). Suppose \(p_i\) is some divisor of \(m\). Take a non-degenerate elliptic quadric \(Q^-(2n-1,q^m)\) in \(\text{PG}(2n-1,q^m)\) with quadratic form \(Q\). The underlying vector space \(V(2n,q^m)\) may be considered as a vector space \(V(2nm/p_i,q^{p_i})\) and a non-degenerate elliptic quadric \(Q_i=Q^-(2mn/p_i,q^{p_i})\) can be constructed in \(\text{PG}(2mn/p_i,q^{p_i})\) using the quadratic form \(\text{Tr}_{\text{GF}(q^m)\to \text{GF}(q^{p_i})}\circ Q\). The quadric \(Q_1\) in \(\text{PG}(2mn-1,q)\) with form \(\text{Tr}_{\text{GF}(q^m)\to \text{GF}(q)}\circ Q\) defines a polarity \(\sigma\) in \(\text{PG}(2mn-1,q)\) and the hyperplane \(P^\sigma\) contains either \[ e_i=\frac{(q^{mn-p_i}-1)(q^{mn-1}+1)}{q-1} \text{ or} e_i'=e_i-q^{mn-1} \] points of \(Q_i\) as \(P\in Q_i\) or \(P\notin Q_i\). Note that as a point set in \(\text{PG}(2mn-1,q)\) the quadric \(Q_i\) has size \[ k_i=\frac{(q^{mn-p_i}-1)(q^{mn}+1)}{q-1}. \] Theorem 1. Let \(m,n\geq 2\) be positive itegers and suppose there exist \(l\) distinct integers \(1=p_1,p_2,\dots,p_l\) such that \(p_i\) divides \(p_{i+1}\) for \(i=1,\dots,l-1\) and \(p_l\) divides \(m\). Then there exists a set of size \(k\) with two intersection numbers \(h\) and \(h'=h-q^{mn-1}\) in \(\text{PG}(2mn-1,q)\) with \(h\) as follows: (i) if \(l\) is even, then \(h=e_l-e_{l-1}+e_{l-2}+\cdots+e_2-e_1\) and \(k=k_l-k_{l-1}+k_{l-2}+\cdots+k_2-k_1\); (ii) if \(l\) is odd, then \(h=e_l-e_{l-1}+e_{l-2}+\cdots+e_3-e_2+e_1\) and \(k=k_l-k_{l-1}+k_{l-2}+\cdots+k_3-k_2+k_1\). If instead of using non-degenerate elliptic quadrics we use differences of non-degeterate hyperbolic quadrics essentially the same result may be proved with \[ h_i=\frac{(q^{mn-p_i}+1)(q^{mn-1}-1)}{q-1}, \quad h_i'=h_i+q^{mn-1} \quad \text{and}\quad k_i'=\frac{(q^{mn-p_i}+1)(q^{mn}-1)}{q-1}. \] Theorem 2. Let \(m\geq 2\) and \(n\geq 1\) be positive itegers and suppose there exist \(l\) distinct integers \(1=p_1,p_2,\dots,p_l\) such that \(p_i\) divides \(p_{i+1}\) for \(i=1,\dots,l-1\) and \(p_l\) divides \(m\). Then there exists a set of size \(k\) with two intersection numbers \(h\) and \(h'=h+q^{mn-1}\) in \(\text{PG}(2mn-1,q)\) with \(h\) as follows: (i) if \(l\) is even, then \(h=h_l-h_{l-1}+h_{l-2}+\cdots+h_2-h_1\) and \(k=k_l'-k_{l-1}'+k_{l-2}'+\cdots+k_2'-k_1'\); (ii) if \(l\) is odd, then \(h=h_l-h_{l-1}+h_{l-2}+\cdots+h_3-h_2+h_1\) and \(k=k_l'-k_{l-1}'+k_{l-2}'+\cdots+k_3'-k_2'+k_1'\).

Country
Australia
Keywords

quadric, Combinatorial structures in finite projective spaces, Quadric, 2-weight Codes, two-weight codes, Theoretical Computer Science, C1, 780100 Non-oriented Research, two-weight code, 230111 Geometry, strongly regular graph, Association schemes, strongly regular graphs, Discrete Mathematics and Combinatorics, Two-weight code, quadrics, Mathematics, strongly regular graphs, Strongly regular graph

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selected citations
These citations are derived from selected sources.
This is an alternative to the "Influence" indicator, which also reflects the overall/total impact of an article in the research community at large, based on the underlying citation network (diachronically).
BIP!Citations provided by BIP!
popularity
This indicator reflects the "current" impact/attention (the "hype") of an article in the research community at large, based on the underlying citation network.
BIP!Popularity provided by BIP!
influence
This indicator reflects the overall/total impact of an article in the research community at large, based on the underlying citation network (diachronically).
BIP!Influence provided by BIP!
impulse
This indicator reflects the initial momentum of an article directly after its publication, based on the underlying citation network.
BIP!Impulse provided by BIP!
3
Average
Average
Average
hybrid