Point-free Construction of Real Exponentiation

TL;DR

This paper develops a constructive, point-free approach to defining real exponentiation and logarithms, enabling geometric constructions in topos theory and contributing to adelic geometry research.

Contribution

It introduces a novel point-free construction of exponentiation and logarithms, with new lifting and gluing techniques in point-free topology, applicable in topos-theoretic contexts.

Findings

Constructed point-free real exponentiation and logarithms.

Developed new lifting and gluing techniques in point-free topology.

Linked properties of rational numbers to real exponentiation.

Abstract

We define a point-free construction of real exponentiation and logarithms, i.e.\ we construct the maps $\exp\colon (0, \infty)\times \mathbb{R} \rightarrow \!(0,\infty),\, (x, \zeta) \mapsto x^\zeta$ and $\log\colon (1,\infty)\times (0, \infty) \rightarrow\mathbb{R},\, (b, y) \mapsto \log_b(y)$, and we develop familiar algebraic rules for them. The point-free approach is constructive, and defines the points of a space as models of a geometric theory, rather than as elements of a set - in particular, this allows geometric constructions to be applied to points living in toposes other than Set. Our geometric development includes new lifting and gluing techniques in point-free topology, which highlight how properties of $\mathbb{Q}$ determine properties of real exponentiation. This work is motivated by our broader research programme of developing a version of adelic geometry via topos theory. In particular, we wish to construct the classifying topos of places of $\mathbb{Q}$, which will provide a geometric perspective into the subtle relationship between $\mathbb{R}$ and $\mathbb{Q}_p$, a question of longstanding number-theoretic interest.