Big Curly braces in Latex with

Question

I am trying to get the equation below to display a big curly braces with 4 conditional equations but my latex is giving me error 'File not found'.

 \documentclass{article}   
 \usepackage{amsmath}
 \begin{equation}
   H^'(\theta)_{mn}=
   \begin{cases}
    \frac{x_i-x_j}{d_{ij}}, & \text{if \theta_n = x_i}.\\
    \frac{x_j-x_i}{d_{ij}}, & \text{if \theta_n = x_j}.\\
    \frac{y_i-y_j}{d_{ij}}, & \text{if \theta_n = y_i}.\\
    \frac{y_j-y_i}{d_{ij}}, & \text{if \theta_n = y_j}.\\
    0, & \text{otherwise}.
   \end{cases}
\end{equation}
\end{document}

Answer 1

Multiple errors (e.g., no \begin{document}, H^' and maths inside \text command arguments) and a style issue. Here's one that works fine for me:

\documentclass{article}   
\usepackage{amsmath}
\begin{document}
 \begin{equation}
   H'(\theta)_{mn}=
   \begin{cases}
    (x_i-x_j)/d_{ij}, & \text{if } \theta_n = x_i.\\
    (x_j-x_i)/d_{ij}, & \text{if } \theta_n = x_j.\\
    (y_i-y_j)/d_{ij}, & \text{if } \theta_n = y_i.\\
    (y_j-y_i)/d_{ij}, & \text{if } \theta_n = y_j.\\
    0, & \text{otherwise}.
   \end{cases}
\end{equation}
\end{document}