The stable Picard group of finite Adams Hopf algebroids with an application to the $\mathbb{R}$-motivic Steenrod subalgebra $\mathcal{A}(1)^{\mathbb{R}}$

TL;DR

This paper analyzes the Picard group of certain Hopf algebroids, particularly computing it for the $ ext{R}$-motivic Steenrod algebra, revealing its structure as isomorphic to $ ext{Z}^4$ with specific generators.

Contribution

It introduces a reduction method via base changes to compute the Picard group of the $ ext{R}$-motivic Steenrod algebra, providing the first explicit description of its structure.

Findings

Picard group of $ ext{A}(1)^{ ext{R}}$ is isomorphic to $ ext{Z}^4$.

Two ranks from motivic grading, one from the algebraic loop functor, one from the $ ext{R}$-motivic joker.

Establishes rigidity of the stable comodule category for finite Adams Hopf algebroids.

Abstract

In this paper, we investigate the rigidity of the stable comodule category of a specific class of Hopf algebroids known as finite Adams, shedding light on its Picard group. Then we establish a reduction process through base changes, enabling us to effectively compute the Picard group of the $\mathbb{R}$-motivic mod $2$ Steenrod subalgebra $\mathcal{A}(1)^{\mathbb{R}}$. Our computation shows that $\operatorname{Pic}(\mathcal{A}(1)^{\mathbb{R}})$ is isomorphic to $\mathbb{Z}^4$, where two ranks come from the motivic grading, one from the algebraic loop functor, and the last is generated by the $\mathbb{R}$-motivic joker $J$.