Explicit Cocycle of the Dedekind-Rademacher Cohomology Class and the Darmon-Dasgupta Measures

Abstract

The work of Darmon, Pozzi, and Vonk has recently shown that the RM-values of the Dedekind-Rademacher cocycle $J_{DR}$ are Gross-Stark units up to a controlled torsion. In the aforementioned work, it is remarked that the measure-valued cohomology class ${\mu }_{DR}$ which underlies $J_{DR}$ is the level 1 incarnation of earlier constructions by Darmon and Dasgupta. In this paper, we make this relationship explicit by computing a concrete cocycle representative of ${\mu }_{DR}$ by tracing the construction of the cohomology class and comparing periods of weight 2 Eisenstein series. While maintaining a global perspective in our computations, we configure the appropriate method of smoothing cocycles which exactly yields the $p$-adic measures of Darmon and Dasgupta when applied to ${\mu }_{DR}$. These methods will also explain the optional degree zero condition imposed in Darmon and Dasgupta's work which was remarked upon in works of Fleischer and Liu as well as Dasgupta and Kakde.

26 pages. Edits to definition of power series ring $P$ and various typo fixes. Version submitted for publication