Splitting conditions in equations
How about the amsmath
environment cases
? Like this:
\documentclass[12pt]{article}
\usepackage{amsmath}
\usepackage{amssymb}
\begin{document}
\begin{equation*}
\rho(X,Y)=
\begin{cases}
\phantom{-}1 & \text{if and only if $\mathbb{P}(Y=\alpha X+\beta)=1$}
\\
&\text{for some real $\alpha$ and $\beta$ with $\alpha >0$,}
\\[4pt]
-1 & \text{if and only if $\mathbb{P}(Y=\alpha X+\beta)=1$}
\\
&\text{for some real $\alpha$ and $\beta$ with $\alpha < 0$.}
\end{cases}
\end{equation*}
\end{document}
If desired, the line spacing between the pair of lines in each two-line condition could be tightened up a bit by using, say, \\[-2pt]
in place of the first and third instance of \\
Please post full documents not fragments, I'd use parbox or aligned
\documentclass{article}
\usepackage{amsmath,amsfonts}
\begin{document}
a
\begin{align*}
\rho(X,Y)&=1&&
\parbox[t]{5cm}{
if and only if $\mathbb P(Y=\alpha X+\beta)=1$\\
for some real $\alpha$ and $\beta$ with $\alpha>0$,}
\\[\jot]
\rho(X,Y)&=-1&&
\parbox[t]{5cm}{if and only if $\mathbb P(Y=\alpha X+\beta)=1$\\
for some real $\alpha$ and $\beta$ with $\alpha<0$.}
\end{align*}
or
\begin{align*}
\rho(X,Y)&=1&&
\begin{aligned}[t]\text{if and only if }\mathbb P(Y=\alpha X+\beta)=1\\
\text{for some real $\alpha$ and $\beta$ with }\alpha>0,
\end{aligned}\\[\jot]
\rho(X,Y)&=-1&&
\begin{aligned}[t]\text{if and only if }\mathbb P(Y=\alpha X+\beta)=1\\
\text{for some real $\alpha$ and $\beta$ with }\alpha<0.
\end{aligned}
\end{align*}
\end{document}
A variant not mentioned in other answers, which uses tabular
.
It has the advantage that you don't need to guess the width of the \parbox
and avoids clumsy \text
macros. The specifier @{}l@{}
means one left aligned column with no padding on either side.
With alignat
you have finer control over the space between the two parts.
\documentclass{article}
\usepackage{amsmath,amssymb}
\renewcommand{\Pr}{\operatorname{\mathbb{P}}} % probability operator
\begin{document}
\begin{alignat*}{2}
\rho(X,Y) &= 1 &\qquad& \begin{tabular}[t]{@{}l@{}}
if and only if $\Pr(Y=\alpha X+\beta)=1$ \\
for some real $\alpha$ and $\beta$ with $\alpha>0$,
\end{tabular}
\\[1ex]
\rho(X,Y) &= -1 && \begin{tabular}[t]{@{}l@{}}
if and only if $\Pr(Y=\alpha X+\beta)=1$ \\
for some real $\alpha$ and $\beta$ with $\alpha<0$.
\end{tabular}
\end{alignat*}
\end{document}