\begin{tabular}{cccc}
		\begin{tikzpicture}
			\draw[fill=black](0,0)circle(2pt) node[below] {\scriptsize $(s,t-1)$};
			\draw[fill=black](0,1)circle(2pt) node[above] {\scriptsize $(s,t)$};
			\draw[->,>=triangle 45, shorten >=2pt] (0,0)--(0,1) ;
			\node[right] at (0,0.5) {$1$};
		\end{tikzpicture}&
		\begin{tikzpicture}
			\draw[fill=black](0,0)circle(2pt) node[below] {\scriptsize $(s,t)$};
			\draw[fill=black](0,1)circle(2pt) node[above] {\scriptsize $(s,t+1)$};
			\draw[->,>=triangle 45, shorten >=2pt] (0,1)--(0,0) ;
			\node[right] at (0,0.5) {$1$};
		\end{tikzpicture}&
		\begin{tikzpicture}	
			\node[color=white] at (0.5,-0.3) {.};
			\draw[fill=black](0,0.5)circle(2pt) node[below] {\scriptsize $(s-1,t)$};
			\draw[fill=black](1,0.5)circle(2pt) node[below] {\scriptsize $(s,t)$};
			\draw[->,>=triangle 45, shorten >=2pt] (0,0.5)--(1,0.5) ;
			\node[above] at (0.5,0.5) {$W(s,t)$};
		\end{tikzpicture}&
		\begin{tikzpicture}	
			\node[color=white] at (0.5,-0.3) {.};
			\draw[fill=black](0,0.5)circle(2pt) node[below] {\scriptsize $(s-1,t)$};
			\draw[fill=black](1,0.5)circle(2pt) node[below] {\scriptsize $(s,t)$};
			\draw[->,>=triangle 45,shorten >=2pt] (1,0.5)--(0,0.5) ;
			\node[above] at (0.5,0.5) {$W(s,t)^{-1}$};
		\end{tikzpicture}
	\end{tabular}