Relationships between p-unit constructions for real quadratic fields

Abstract

Let $K$ be a real quadratic field and let $p$ be a prime number which is inert in $K$. Let $K_p$ be the completion of $K$ at $p$. In a previous paper, we constructed a $p$-adic invariant $u_C\in K_p$, and we proved a $p$-adic Kronecker limit formula relating $u_C$ to the first derivative at $s=0$ of a certain $p$-adic zeta function. By analogy with the $p$- adic Gross-Stark conjectures, we conjectured that $u_C$ is a $p$-unit in a suitable narrow ray class field of $K$. Recently, Dasgupta has proposed an exact $p$-adic formula for the Gross-Stark units of an arbitrary totally real number field. In our special setting, i.e., where one deals with a real quadratic number field, his construction produces a $p$-adic invariant $u_D\in K_p$ . In this paper we show precise relationships between the $p$-adic invariants $u_C$ and $u_D$. In order to do so, we extend Dasgupta's construction of $u_D$ to a broader setting.