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The following LaTeX document:

\begin{align*}
    p{(A,RM,OL,O,LC)}   &=p(A)\;p(RM\mid A)\;p(OL\mid A,RM)\;p(O \mid A, RM, OL)    \\ & 
    \; \; \; \;p(LC\mid A, RM, OL,O) \label{eq:JPExample}
\end{align*}

does not display the reference \ref{eq:JPExample}. Why?

  • Welcome to TeX.SX! A tip: If you indent lines by 4 spaces, they'll be marked as a code sample. You can also highlight the code and click the "code" button (with "{}" on it). – DG' Jan 26 '15 at 9:03
  • 1
    Welcome to TeX.SX! Please make your code compilable (if possible), or at least complete it with \documentclass{...}, the required \usepackage's, \begin{document}, and \end{document}. – mvkorpel Jan 26 '15 at 9:12
  • 3
    You are trying to refer to an equation that has no number. It makes no sense. Change align* to align then it works. – user11232 Jan 26 '15 at 9:16
  • ok. :) i am pretty new in latex :P but thanks for editing ;) – Rosa Jan 26 '15 at 13:28
2

Your alignment is not good. Here is how I would do it:

\documentclass{article}

\usepackage{amsmath}

\begin{document}

\begin{align}
  p(A,RM,OL,O,LC)
  ={}& p(A) \cdot p(RM \mid A) \cdot p(OL \mid A,RM) \nonumber\\
     &  \cdot p(O \mid A,RM,OL) \cdot p(LC \mid A,RM,OL,O). \label{eq:JPExample}
\end{align}
Here is equation~\eqref{eq:JPExample}.

\end{document}

output1

or

\documentclass{article}

\usepackage{amsmath}

\begin{document}

\begin{equation}
\begin{split}
  p(A,RM,OL,O,LC)
  ={}& p(A) \cdot p(RM \mid A) \cdot p(OL \mid A,RM) \\
     &  \cdot p(O \mid A,RM,OL) \cdot p(LC \mid A,RM,OL,O). \label{eq:JPExample}
\end{split}
\end{equation}
Here is equation~\eqref{eq:JPExample}.

\end{document}

output2

Notice the difference in the tag placement between the two approaches.

Also, I've

  • removed the redundant brackets in p{(...)},
  • put multiplications signs between each factor,
  • moved one of the factor from the first line to the latter.
  • 1
    you don't need gather; in this context, equation would be better. (aligned would also work as an alternative to split.) – barbara beeton Jan 26 '15 at 14:14
4

You can only reference equations which also have a number. Since you are using the align* environment, your equation does not have a number and hence cannot be referenced. In the align environment (without the *) they get a number and can be referenced.

Since you have multiple lines and probably only want one of these lines to have a number and to be referenced, you can use the \nonumber command from the amsmath package to prevent those lines from getting a number.

In the example below, the first equation is assigned numbering "(1)" and can be referenced. The second equation is not assigned any number and cannot be referenced.

\documentclass[a4paper,11pt]{report}
\usepackage{amsmath}  %align environment and equations withount unmbering

\begin{document}
  \begin{align}
    E & = m \cdot c^2
    \label{eq:einstein}
    \\
    c^2 & = a^2 + b^2 
    \nonumber    
  \end{align}
  This is a reference to equation \eqref{eq:einstein}.
 %This is a reference to equation (1).
\end{document}

MWE example for equations with/without reference

Note: The \eqref{...} command produces references enclosed in brackets as "equation (1)" compared to "equation 1" using the \ref{...} command.

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