Coding an equation with description

Question

I want to code this equation in LaTeX:

I'd like to know how to code the equation and the description below it with a reference as well.

Answer 1

Some more flair in managing the "where" clause, and formatting of the conditions:

\documentclass{article}
\usepackage{amsmath}% http://ctan.org/pkg/amsmath
\usepackage{array}% http://ctan.org/pkg/array
\begin{document}

\begin{gather}
  P_{xi}=\overline{U}_x+\sigma_x
         \frac{\sum_k^{Nu}D_{kx}\times\left(\frac{S_{ki}-\overline{U}_k}{\sigma_k}\right)}
              {\sum_k^{Nu}D_{kx}},
\intertext{Where:}
  \begin{tabular}{>{$}r<{$}@{\ :\ }l}
    P_{xi} & is the predicted rate for user~$x$ on item~$i$ \\
    S_{ki} & is the rate of song~$i$ given by user~$k$ \\
    D_{kx} & the correlation between user~$x$ and user~$k$ \\
    \overline{U}_x & the average rate over user~$x$ \\
    \overline{U}_k & the average rate over user~$k$ \\
    \sigma_x & is the standard deviation of all the rates of user~$x$
  \end{tabular}\nonumber
\end{gather}

\end{document}​

Answer 2

Even though your question was thoroughly answered here's my solution, it differs only in not using a table but a list:

\documentclass{article}

\usepackage{amsmath}
\usepackage{amssymb}

\usepackage{enumitem}

\begin{document}

\[ P_{xi}=\overline{U_{x}}+\sigma_{x}\frac{\sum^{Nu}_{k}D_{kx}\times 
    \left( \frac{S_{ki}-\overline{U_{k}}}{\sigma_{k}}\right)}{\sum^{Nu}_{k}D_{kx}} \]
Where:
\begin{itemize}[label=]
    \item $P_{xi}$: is the predicted rate for user $x$ on item $i$
    \item $S_{ki}$: is the rate of song $i$ given by user $k$
    \item $\overline{U_{x}}$: is the average rate of user $x$
    \item $\overline{U_{k}}$: is the average rate of user $k$
    \item $\sigma_{x}$: is the standard deviation of all the rates of user $x$
\end{itemize}

\end{document}

Which produces the following result:

Answer 3

You can add the line over the variables using \overline. The text below can be done e.g. using a tabular.

\documentclass{article}
\usepackage{amsmath}
\begin{document}

\begin{gather}
    P_{xi} = \overline{U_{x}} + \sigma_{x} \frac
    {\sum_k^{Nu} D_{kx} \times \left( \frac {(S_{ki}-\overline{U_k})}{\sigma_k} \right) }
    {\sum_k^{Nu} D_{kx}},
\end{gather}
Where:\\
\hspace*{3em}
\begin{tabular}{rl}
    $P_{xi}$:& is the predicted rate for user $x$ on item $i$. \\
    $S_{ki}$:& is the rate of song $i$ given by user $k$. \\
    ... & ... \\
\end{tabular}

\end{document}

You can change the space between the columns in the tabular by adding @{\hspace{<your space>}} between the r and l column type specifier.

Answer 4

If you want to create a numbered equation as in your example, put the math part in an equation environment and add a \label that can be used to reference the equation later (with \ref{}). The LaTeX code of your equation could look like this:

\begin{equation}
  \label{eq:mylabel}
  P_{xi}=\overline{U}_x+\sigma_x
         \frac{\sum_k^{Nu}D_{kx}\times\left(\frac{S_{ki}-\overline{U}_k}{\sigma_k}\right)}
              {\sum_k^{Nu}D_{kx}},
\end{equation}

Of course, you can tweak it to your liking, e.g. put the sum limits above/below the sum (append \limits to the \sum command) or use bigger sum symbols. You should have a look into one of the many LaTeX books available to familiarize yourself with the system.