# Aligning multiple inequality symbols

I have a question about how to align multiple symbols for a set of inequalities to make it look clean. This is what I have at the moment:

\begin{align*}
|f(x)-f(a)| &< f(a) \\
-f(a) < f(x) - f(a) &< f(a) \\
-f(a) + f(a) < f(x) &< f(a) + f(a) \\
0 < f(x) &< 2f(a) \\
\implies f(x) &> 0.
\end{align*}


Which only aligns the right inequality signs with each other: What I would like to accomplish is similar to the result one might get when using the alignat environment. Ideally, I would like to essentially create 3 columns with the inequality signs acting as separators - but I would like the middle column to be center justified.

Also note that the top row only has one inequality sign - I would like that to be aligned with all the signs on the right. How might I go about accomplishing this?

• Related/duplicate: Align two inequalities – Werner Mar 22 '17 at 10:23
• Is there a reason why you don't just use alignat? – Skillmon Mar 22 '17 at 10:26
• It doesn't center the middle column... Was wondering if there is a clean way to do that. – CoffeeDonut Mar 22 '17 at 10:27

Like this? With use ow array it is simple:

\documentclass{article}
\usepackage{mathtools}

\begin{document}
$\setlength\arraycolsep{1pt} \begin{array}{rcccl} & ~ & f(x)-f(a)| & < & f(a) \\ -f(a) & < & f(x) - f(a) & < & f(a) \\ -f(a) + f(a) & < & f(x) & < & f(a) + f(a) \\ 0 & < & f(x) & < & 2f(a) \\ & ~ &\implies f(x)& > & 0 \end{array}$
\end{document}

• Well, it is certainly prettier than what I have. Thanks for the help! – CoffeeDonut Mar 22 '17 at 10:31

A solution with alignat and the eqparbox package. I took the opportunity do define an \eqmathbox command: its optional argument is a tag (M by default), and its mandatory argument is in mathmode, display style. All \eqmathboxes sharing the same tag will have their contents centred in a box of width the largest contents width. I also defined an \abs command, which adds an implicit pair of \left\lvert … \right\rvert around its argument in its starred version.

\documentclass{article}
\usepackage{mathtools}
\DeclarePairedDelimiter\abs\lvert\rvert
\usepackage{eqparbox}
\newcommand\eqmathbox[M]{\eqmakebox[#1]{$\displaystyle#2$}}

\begin{document}

\begin{alignat*}{2}
& \phantom{{}<{}} & \eqmathbox{\abs{ f(x)-f(a)}} & < f(a) \\
-f(a) & < & \eqmathbox{f(x)-f(a)} & < f(a) \\%
-f(a) + f(a) & < & \eqmathbox{f(x)} & < f(a) + f(a) \\
0 & < & \eqmathbox{f(x)} & < 2f(a) \\
& & \implies f(x) & > 0
\end{alignat*}

\end{document} 