How to force a linebreak in itemization?

Question

I have a proposition which consists of two itemized bullets. I want both to be displayed in an offset way, but the first one keeps getting displayed as a continuation of the Proposition in bold.

How to get what I want?

This is my code.

\documentclass[12pt]{amsart}
\usepackage{amsthm}
\usepackage{amssymb}
\usepackage{latexsym}
\usepackage{url}
\title{whatever}
\newtheorem{thm}{Theorem}[section]
\newtheorem{prop}[thm]{Proposition}
\begin{document}
\begin{prop}
This is the usual display.
\end{prop}
\begin{prop}
\label{prop:twocases}
\begin{itemize}
\mbox{} \item If $b_{1} \leq \sqrt{x}$, then: $\epsilon_{i}=\epsilon_{i}^{+}, i
\in \{1,2,3\}.$
\item If $b_{1} > \sqrt{x}$, then either $\epsilon_{i}=\epsilon_{i}^{+}, i \in
\{1,2,3\}$ or $\epsilon_{i}=\epsilon_{i}^{-}, i \in \{1,2,3\}.$
\end{itemize}
\end{prop}
\end{document}

Answer 1

You put the \mbox{} in the wrong place:

 \mbox{}\begin{itemize}

Is where it should go.