Marshall’s Quotient and the Arason–Pfister Hauptsatz for Reduced Special Groups

We provide a new proof of the Arason–Pfister Hauptsatz (APH) in the setting of reduced special groups, as developed by Dickmann and Miraglia. Our approach avoids the use of Boolean invariants and instead relies on a construction inspired by Marshall’s quotient, suitably adapted to the context of special groups. We establish structural properties of this quotient and show that it generalizes the Pfister quotient by a Pfister subgroup. Using this framework, we define iterated quadratic extensions of special groups and develop a theory of Arason–Pfister sequences. These tools allow us to prove that any anisotropic form φ ∈ I n ( G ) over a reduced special group G satisfies the inequality dim ( φ ) ≥ 2 n , where I n ( G ) denotes the n -th power of the fundamental ideal of the Witt ring of G . Our methods are purely algebraic and internal to the theory of special groups, contributing with novel tools to the categorical study of abstract theories of quadratic forms.