[tex] equation number is one , how to reference it

\documentclass[12pt,a4paper]{article}

\renewcommand{\figurename}{圖}

\usepackage[dvips]{graphicx}

\begin{document}

\begin{equation}\label{weight_value}

w(i,j)= \left\{

\begin{array}{lll}

0 & \mbox{(i=j)}\\[2pt]

\frac{1}{H(i,j)} &\mbox{(i $\neq$ j, AP(i) and AP(j) are adjacent)}\\[2pt]

\infty & \mbox{(AP(i) and AP(j) are not adjacent)}

\end{array}\right.

\end{equation}

%\left\{ 這個要和 \right. 配合 這裡要有一個點 \end{equation}

呈現依w(i,j).eq (\Z\ref{weight_value}) 秀出權重值在ap(i)和ap(j)之間.

%如果第一次compile,有出現??.那麼請在compile第二次,3q.

\end{document}