conditional equation in latex [duplicate]

Question

how can I write this formula by latex

Answer 1

One way is to use:

\begin{align}
p_{i \, j} = \begin{cases} \frac{1}{|p_{i}|} & \text{if $P_{i}$ links to $P_{j}$} \\
0 & \text{otherwise} \end{cases}
\end{align}

where align is used for spacing around the equation(s) or an inline version is:

 $$ p_{i \, j} = \begin{cases} \frac{1}{|p_{i}|} & \text{if $P_{i}$ links to $P_{j}$} \\
 0 & \text{otherwise} \end{cases} $$

The results can be viewed as (in their respective order):

Answer 2

With use of cases* and/or dcases?defined in the mathtools package and defining \abs{...} as pair of delimiters for absolute value:

\documentclass[12pt]{article}
\usepackage{mathtools}
\DeclarePairedDelimiter\abs{\lvert}{\rvert}

\begin{document}
\begin{equation}
p_{ij} = \begin{cases*} 
    \dfrac{1}{\abs{p_{i}}}   & if $P_{i}$ links to $P_{j}$ \\
                    0       & otherwise 
             \end{cases*}
\end{equation}
or
\begin{equation}
p_{ij} = \begin{dcases*} 
    \frac{1}{\abs{p_{i}}}   & if $P_{i}$ links to $P_{j}$ \\
                    0       & otherwise 
             \end{dcases*}
\end{equation}

\end{document}