\documentclass{article}

\usepackage{a4wide}
\usepackage{amssymb}
\usepackage{amsmath}

\newcommand{\ra}{\rightarrow}
\newcommand{\un}{\text{ \textsf{U} }}
\newcommand{\ev}{\lozenge}
\newcommand{\al}{\square}
\newcommand{\nx}{\bigcirc}

\begin{document}
\begin{enumerate}
	\item
	\item
	\item
	\item\strut\\
		$\begin{array}{ll}
			\phi_1=\ev\al c & 
				s_2\ra s_4\overline{\ra s_5}\\
			\phi_2=\al\ev c &
				s_2\ra s_4\overline{\ra s_5}\\
			\phi_3=\nx\neg c\rightarrow\nx\nx c &
				s_2\ra s_4\overline{\ra s_5}\\
			\phi_4=\al a & 
				\text{Impossible. $a$ only holds in $s_1$ and in $s_5$.}\\
			& \text{$s_1$ we can only leave through a state where $\neg a$}\\
			& \text{$s_5$ we can not reach through $s_1$ without passing
				through a $\neg a$ state}\\
			\phi_5=a\un\al(b\vee c) &
				s_1\ra s_4\overline{\ra s_5}\\
			\phi_6=(\nx\nx b)\un(b\vee c) &
				s_2\ra s_4\overline{\ra s_5}
		\end{array}$
\end{enumerate}
\end{document}
