Defining a phrase containing math and text usable in both math and text mode

Question

I would like to define a phrase once that contains both math and text, and be able to use usable in both math and text mode. For example:

\newcommand{\Constraint}{\ensuremath{x \neq 0} \text{ and } \ensuremath{x \neq -1}}

This works fine in math mode, but leaves too much space in text mode. Of course:

\newcommand{\Constraint}{\ensuremath{x \neq 0} \text{and} \ensuremath{x \neq -1}}

works fine in text mode, but not in math mode.

Well that led me to:

\newcommand{\Space}{\ifmmode{ }\else{}\fi}%
\newcommand{\Constraint}{\ensuremath{x \neq 0} \text{\Space and\Space} \ensuremath{x \neq -1}}

which of course is a brilliant solution, except for the fact that it only yields correct results in text mode!! I attempted to replace the { } with various math spacing (\;, and \hspace{1ex}) and they had no effect in math mode.

The one case that does yield correct spacing in both is using an \mbox as in:

\newcommand{\Constraint}{\mbox{\ensuremath{x \neq 0}  and \ensuremath{x \neq -1}}}

but then the text can not be split across line boundaries so is not good enough.

Below are my various failed attempts commented out. The code as below yields:

\documentclass{article}
\usepackage{amsmath}
\usepackage{xspace}
\usepackage{showframe}

%%%% Math Mode: Works fine
%%%% Text Mode: Too much space before and after "and"
%\newcommand{\Constraint}{\ensuremath{x \neq 0} \text{ and } \ensuremath{x \neq -1}}

%%%% Math Mode: No space before and after "and"
%%%% Text Mode: Works fine 
%\newcommand{\Constraint}{\ensuremath{x \neq 0} \text{and} \ensuremath{x \neq -1}}


%%%% Math Mode: Works fine
%%%% Text Mode: Too much space before and after "and"
%\newcommand{\Constraint}{\ensuremath{x \neq 0} \text{~and~} \ensuremath{x \neq -1}}


%%%% Math Mode: missing space after "and"
%%%% Text Mode: Too much space before and after "and"
%\newcommand{\Constraint}{\ensuremath{x \neq 0} \text{~and\xspace} \ensuremath{x \neq -1}}

%%%% Math Mode: missing spaces before and after
%%%% Text Mode: Works fine.
\newcommand{\Space}{\ifmmode{ }\else{}\fi}%
\newcommand{\Constraint}{\ensuremath{x \neq 0} \text{\Space and\Space} \ensuremath{x \neq -1}}

\begin{document}
\begin{align*}
    \frac{2x}{x+1} &= \frac {2x-1}{x}\\
    \implies 2x  (x)   &= (x+1) (2x-1) \qquad\Constraint
\end{align*}
%
In a short sentence note the constraint: \Constraint.

In a long sentence note the above the constraint must be satisfied: $\Constraint$.
\end{document}

Answer 1

Edit: Next try: take the spaces outside \text

\newcommand\Space{\penalty\z@\hspace{1ex}}
\newcommand{\Constraint}{\ensuremath{x \neq 0\Space\text{and}\Space x \neq -1}}

I'd have expected there to be a breakable space for math mode already, so maybe I'm still getting something wrong...

Answer 2

The only thing to do is to say

\newcommand{\Constraint}{%
  \relax\ifmmode
    x\neq 0 \text{ and } x\neq 1
  \else
    $x\neq 0$ and $x\neq 1$
  \fi}

If one has loaded babel, then \textormath could be used instead of the conditional, but it's the same concept.

Why not \ensuremath{x\neq 0 \text{ and } x\neq1}?

First of all because it uses \ensuremath. :) Seriously, because the only break points would be after the relation symbols \neq, which is where the break is less preferable. The problem is, in fact, that \text creates a box.

Add ties and \nobreak where it seems suitable. And be careful in defining such abbreviations (that will be mysterious when, a couple of months from now, you'll be again reading the document).

How to avoid repeating the text? My solution would be to spell the text without resorting to mysterious abbreviations. Or, if you insist,

\newcommand{\ensuretext}[1]{\ifmmode#1\else\text{#1}\fi}
\newcommand{\Constraint}{%
  \ensuremath{x\neq 0}\ensuretext{ and }\ensuremath{x \neq 1}}

which is worse that specifying the text twice.

Answer 3

Replace ensuremath by the \( \).

\documentclass{article}
\usepackage{amsmath}
\usepackage{xspace}
\newcommand{\Constraint}{\mbox{\(x \neq 0\)  and \(x \neq -1\)}} %<---- replace \ensure math{} by \( \).
%------------------------------------
\begin{document}
\begin{align*}
    \frac{2x}{x+1} &= \frac {2x-1}{x}\\
    \implies 2x  (x)   &= (x+1) (2x-1) \qquad\Constraint
\end{align*}
%
Note in the above the constraint: \Constraint.
\end{document}