# Invertible $K(2)$-Local $E$-Modules in $C_4$-Spectra

## TL;DR

This paper computes the Picard group of $K(2)$-local module spectra over a specific ring spectrum related to Morava $E$-theory, revealing its structure and new subgroup components.

## Contribution

It provides the first complete calculation of the Picard group for $K(2)$-local $E$-modules in genuine $C_4$-spectra, including new subgroup identifications.

## Key findings

- Identified a cyclic subgroup of order 32 in the Picard group.
- Discovered a subgroup of order 2 generated by a specific spectrum shift.
- Computed the $RO(C_4)$-graded Mackey functor homotopy fixed point spectral sequence.

## Abstract

We compute the Picard group of the category of $K(2)$-local module spectra over the ring spectrum $E^{hC_4}$, where $E$ is a height 2 Morava $E$-theory and $C_4$ is a subgroup of the associated Morava stabilizer group. This group can be identified with the Picard group of $K(2)$-local $E$-modules in genuine $C_4$-spectra. We show that in addition to a cyclic subgroup of order 32 generated by $ E\wedge S^1$ the Picard group contains a subgroup of order 2 generated by $E\wedge S^{7+\sigma}$, where $\sigma$ is the sign representation of the group $C_4$. In the process, we completely compute the $RO(C_4)$-graded Mackey functor homotopy fixed point spectral sequence for the $C_4$-spectrum $E$.