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

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 $\mathrm{Pic}(\mathcal{A}(1{)}^{\mathbb{R}})$ 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 $J$.

18 pages, 4 figures