TY - JOUR
T1 - A relative m-cover of a Hermitian surface is a relative hemisystem
AU - Bamberg, John
AU - Lee, Melissa
N1 - Funding Information:
The authors are indebted to Michael Giudici for many discussions on the material in this work. The first author acknowledges the support of the Australian Research Council Future Fellowship FT120100036. The second author acknowledges the support of a Hackett Postgraduate Scholarship. The authors also thank Daniel Horsley for his mention of an elementary result during his talk at Combinatorics 2016, which was instrumental in the proof of the main result of this paper.
Publisher Copyright:
© 2017, Springer Science+Business Media New York.
PY - 2017/6/1
Y1 - 2017/6/1
N2 - An m-cover of the Hermitian surface H (3 , q2) of PG (3 , q2) is a set S of lines of H (3 , q2) such that every point of H (3 , q2) lies on exactly m lines of S, and 0 < m< q+ 1. Segre (Annali di Matematica Pura ed Applicata Serie Quarta 70:1–201, 1965) proved that if q is odd, then m= (q+ 1) / 2 , and called such a set S of lines a hemisystem. Penttila and Williford (J Comb Theory Ser A 118(2):502–509, 2011) introduced the notion of a relative hemisystem of a generalised quadrangle Γ with respect to a subquadrangle Γ′: a set of lines R of Γ disjoint from Γ′ such that every point P of Γ\ Γ′ has half of its lines (disjoint from Γ′) lying in R. In this paper, we provide an analogue of Segre’s result by introducing relative m-covers of generalised quadrangles of order (q2, q) with respect to a subquadrangle and proving that m must be q / 2 when the subquadrangle is doubly subtended. In particular, a relative m-cover of H (3 , q2) with respect to a symplectic subgeometry W (3 , q) is a relative hemisystem.
AB - An m-cover of the Hermitian surface H (3 , q2) of PG (3 , q2) is a set S of lines of H (3 , q2) such that every point of H (3 , q2) lies on exactly m lines of S, and 0 < m< q+ 1. Segre (Annali di Matematica Pura ed Applicata Serie Quarta 70:1–201, 1965) proved that if q is odd, then m= (q+ 1) / 2 , and called such a set S of lines a hemisystem. Penttila and Williford (J Comb Theory Ser A 118(2):502–509, 2011) introduced the notion of a relative hemisystem of a generalised quadrangle Γ with respect to a subquadrangle Γ′: a set of lines R of Γ disjoint from Γ′ such that every point P of Γ\ Γ′ has half of its lines (disjoint from Γ′) lying in R. In this paper, we provide an analogue of Segre’s result by introducing relative m-covers of generalised quadrangles of order (q2, q) with respect to a subquadrangle and proving that m must be q / 2 when the subquadrangle is doubly subtended. In particular, a relative m-cover of H (3 , q2) with respect to a symplectic subgeometry W (3 , q) is a relative hemisystem.
KW - Generalised quadrangle
KW - Hermitian surface
KW - Relative hemisystem
UR - http://www.scopus.com/inward/record.url?scp=85010736895&partnerID=8YFLogxK
U2 - 10.1007/s10801-017-0739-5
DO - 10.1007/s10801-017-0739-5
M3 - Article
AN - SCOPUS:85010736895
SN - 0925-9899
VL - 45
SP - 1217
EP - 1228
JO - Journal of Algebraic Combinatorics
JF - Journal of Algebraic Combinatorics
IS - 4
ER -