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Generation of defect groups and Galois action for $\mathrm{Sp}_4(q)$

Bjorn Cattell-Ravdal, A. A. Schaeffer Fry and Nicole Venner

Vol. 19 (2026), No. 3, 423–442
Abstract

The group is a well-known algebraic object and one of the most fundamental structures within mathematics. For a given group, we may correspond the elements to those of a group of invertible matrices, forming a matrix representation of the group. Associated with these representations are characters, obtained by taking the traces of each matrix. Studying these characters, we find that they contain much information about their underlying group. One of the main goals in character theory is to determine what properties about the group can be determined by looking at its characters. We will discuss several recent conjectures on this topic that study properties of certain subgroups known as defect groups. In particular, we prove some such conjectures regarding generation properties of defect 2-groups for the symplectic group Sp 4(q), which is a particular group of 4 × 4 matrices over any finite field 𝔽q of odd characteristic.

Keywords
McKay–Navarro conjecture, Alperin–McKay conjecture, Alperin–McKay–Navarro conjecture, height-zero characters, Galois action, defect groups
Mathematical Subject Classification
Primary: 20C15, 20C33
Secondary: 20C20
Milestones
Received: 3 August 2024
Accepted: 21 December 2024
Published: 12 June 2026

Communicated by Scott T. Chapman
Authors
Bjorn Cattell-Ravdal
Department of Mathematics and Statistics
Metropolitan State University Denver
Denver, CO
United States
A. A. Schaeffer Fry
Department of Mathematics
University of Denver
Denver, CO
United States
Nicole Venner
Department of Mathematics and Statistics
Metropolitan State University Denver
Denver, CO
United States