A Generalization of Strassen’s Theorem on Preordered Semirings

Given a commutative semiring with a compatible preorder satisfying a version of the Archimedean property, the asymptotic spectrum, as introduced by Strassen (J. reine angew. Math. 1988), is an essentially unique compact Hausdorff space together with a map from the semiring to the ring of continuous functions. Strassen’s theorem characterizes an asymptotic relaxation of the preorder that asymptotically compares large powers of the elements up to a subexponential factor as the pointwise partial order of the corresponding functions, realizing the asymptotic spectrum as the space of monotone semiring homomorphisms to the nonnegative real numbers. Such preordered semirings have found applications in complexity theory and information theory. We prove a generalization of this theorem to preordered semirings that satisfy a weaker polynomial growth condition. This weaker hypothesis does not ensure in itself that nonnegative real-valued monotone homomorphisms characterize the (appropriate modification of the) asymptotic preorder. We find a sufficient condition as well as an equivalent condition for this to hold. Under these conditions the asymptotic spectrum is a locally compact Hausdorff space satisfying a similar universal property as in Strassen’s work.

Rational Cup Product and Algebraic K0-Groups of Rings of Continuous Functions

A connected space is called a C0-space if its rational cup product is trivial. A characterizing property of C0-spaces is obtained. This property is used to calculate the algebraic K0-group K0(C𝔽(X)) of the ring of continuous functions for infinite-dimensional complexes X.

Adelic descent theory

A result of André Weil allows one to describe rank $n$ vector bundles on a smooth complete algebraic curve up to isomorphism via a double quotient of the set $\text{GL}_{n}(\mathbb{A})$ of regular matrices over the ring of adèles (over algebraically closed fields, this result is also known to extend to $G$-torsors for a reductive algebraic group $G$). In the present paper we develop analogous adelic descriptions for vector and principal bundles on arbitrary Noetherian schemes, by proving an adelic descent theorem for perfect complexes. We show that for Beilinson’s co-simplicial ring of adèles $\mathbb{A}_{X}^{\bullet }$, we have an equivalence $\mathsf{Perf}(X)\simeq |\mathsf{Perf}(\mathbb{A}_{X}^{\bullet })|$ between perfect complexes on $X$ and cartesian perfect complexes for $\mathbb{A}_{X}^{\bullet }$. Using the Tannakian formalism for symmetric monoidal $\infty$-categories, we conclude that a Noetherian scheme can be reconstructed from the co-simplicial ring of adèles. We view this statement as a scheme-theoretic analogue of Gelfand–Naimark’s reconstruction theorem for locally compact topological spaces from their ring of continuous functions. Several results for categories of perfect complexes over (a strong form of) flasque sheaves of algebras are established, which might be of independent interest.

The super socle of the ring of continuous functions

We introduce and study the concept of the super socle of

A note on the isomorphism problem for SK[G]

Let G be an infinite abelian p-group and let K be a field of characteristic ≠ p. Let K[G] be the set of all sums Σg∈Gagg where the ag are in K, and all but finitely many ag are 0. Then K[G] is a K-algebra, with multiplication induced by the group multiplication on G.If G is countable, then the isomorphism type of K[G] has been completely described by S. D. Berman [1]. If G is a direct sum of countable groups, one can also describe K[G], as K[⊕iGi] ≃ ⊗iK[Gi]. If K contains all pn-th roots of unity, then K[G] is isomorphic to the ring of continuous functions from a Boolean space X to the field K with the discrete topology. In that case, the group UK[G] of invertible elements of K[G] is isomorphic to the direct sum of ∣G∣ copies of K×. More generally, if K is of the second kind with respect to p (see below for the definition), the group UK[G] has a simple description.Consider the subgroup SK[G] of elements Σgagg which have order a power of p and such that Σg ag = 1. This group is of course much simpler than UK[G]. Classifying SK[G] up to isomorphism reduces to the case where G has no element of infinite height, see [7]. If G is a direct sum of cyclic groups then the isomorphism type of SK[G] has been completely determined, in [2, 3, 7, 8]. The aim of this note is to show that a similar result is in general not possible for uncountable G. We use an invariant Γ associated to abelian groups, and for any regular uncountable cardinal κ, exhibit 2κ groups G for which Γ(G) = Γ(SK[G]) are pairwise distinct. Our work is based on a construction of Shelah [9], who constructed 2κ non-isomorphic abelian p-groups of cardinality κ for κ an uncountable regular cardinal.