We develop a higher semiadditive version of Grothendieck-Witt theory. We then apply the theory in the case of a finite field to study the higher semiadditive structure of the K(1)local sphere at the prime 2. As a further application, we compute and clarify certain power operations in the homotopy of the K(1)-local sphere.Here, the first map is (the inverse of) the equivalence resulting from the higher semiadditive structure, and the second map is induced by the map A → pt.Remark 1.0.2. The collection of these ∫ A maps, together with corresponding restriction maps, endow X with a structure known as an m-commutative monoid (with 0-commutative monoids being just ordinary commutative monoids). First described systematically by Harpaz [Har20], this m-commutative monoid structure for X is captured by a functor X (−) : Span(S m ) op → Sp given by A → X A . Here, Span(S m ) is the ∞-category of m-truncated π-finite spaces and spans. In fact, we will work in a p-typical setting, where one further restricts to m-truncated π-finite spaces with homotopy groups of p-power order. We refer the reader to Section 2 for a more careful discussion of (p-typical) m-commutative monoids.
Semiadditive cardinalityThis paper is focused on one part of this m-commutative monoid structure: Definition 1.1.1 ([CSY20] Definition 2.1.5, [HL13] Notation 5.1.7). Let C be an m-semiadditive ∞-category, let X ∈ C and let A be an m-truncated π-finite space. Then let |A| X : X → X be the endomorphism of X defined by the compositewhere ∆ A is the canonical diagonal map induced by A → pt. As X varies, these maps assemble to a natural transformation |A| : Id C → Id C , which we will refer to as the cardinality of A.This terminology is justified by the following example:Example 1.1.2. In the special case where A is discrete, the endomorphism |A| is given by multiplication by the cardinality of the finite set A.