3

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: enter image description here

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?

3
  • Related/duplicate: Align two inequalities
    – Werner
    Commented Mar 22, 2017 at 10:23
  • Is there a reason why you don't just use alignat?
    – Skillmon
    Commented Mar 22, 2017 at 10:26
  • It doesn't center the middle column... Was wondering if there is a clean way to do that. Commented Mar 22, 2017 at 10:27

2 Answers 2

5

Like this?

enter image description here

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}
1
  • Well, it is certainly prettier than what I have. Thanks for the help! Commented Mar 22, 2017 at 10:31
4

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[2][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} 

enter image description here

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