How do I add a list of definitions following “where?” [closed]

Question

I have one small equation with two supportive terms to explain the equation.

\begin{equation}
[\mathrm{d}[\mathrm{i}], \mathrm{a}[\mathrm{i}]]=\mathrm{f}_{i}(\mathrm{M}[\mathrm{i}] \cdot \mathrm{f})
\end{equation}


where $\mathrm{d}[\mathrm{i}] \in \mathbb{R}^{B \times n_{d}}$ and  $\mathrm{a}[\mathrm{i}] \in \mathbb{R}^{B \times n_{a}}$ \vspace{0.5\baselineskip}

That is not looking neat and good in paper. I am using mathptmx but not getting good result. How I can make it to look better.

Edit

\begin{equation}
[d[i], a[i]] = f_{i}(M[i]\cdot f)
\end{equation}


where $d[i] \in \mathbb{R}^{B \times n_{d}}$ and  $a[i] \in \mathbb{R}^{B \times n_{a}}$ \vspace{0.5\baselineskip}

Answer 1

This will come down to personal preference, but here’s what I would suggest.

First, you can use \shortintertext{where} from mathtools within a split to introduce the definitions.

Second, I took the liberty of making the outer brackets and parentheses one size larger than the inner brackets.

\documentclass{article}
\usepackage{mathtools} % For \shortintertext
\usepackage{unicode-math} % For \setmathfont

\defaultfontfeatures{Scale = MatchLowercase, Ligatures = TeX }
\setmainfont{TeX Gyre Termes}[Scale=1.0] % Modern version of times or newtxtext
\setmathfont{TeX Gyre Termes Math} % Modern version of mathptmx or newtxmath

%% To format the MWE for TeX.SX:
\usepackage[paperwidth=10cm]{geometry}
\pagestyle{empty}

\begin{document}
\begin{equation}
  \begin{split}
    \bigl[ d[i], a[i] \bigr] = f_{i}\bigl( M[i]\cdot f \bigr)
    \shortintertext{where}
    \qquad d[i] \in \mathbb{R}^{B \times n_{d}} \textnormal{ and } a[i] \in \mathbb{R}^{B \times n_{a}}
  \end{split}
\end{equation}
\end{document}

This solution also preserves alignment points, so you could align a list of definitions, like so:

\documentclass{article}
\usepackage{mathtools} % For \shortintertext
\usepackage{unicode-math} % For \setmathfont

\defaultfontfeatures{Scale = MatchLowercase, Ligatures = TeX }
\setmainfont{TeX Gyre Termes}[Scale=1.0] % Modern version of times or newtxtext
\setmathfont{TeX Gyre Termes Math} % Modern version of mathptmx or newtxmath

%% To format the MWE for TeX.SX:
\usepackage[paperwidth=10cm]{geometry}
\pagestyle{empty}

\begin{document}
\begin{align}
  \begin{split}
    \qquad \bigl[ d[i], a[i] \bigr] &= f_{i}\bigl( M[i]\cdot f \bigr)
    \shortintertext{where}
    d[i] &\in \mathbb{R}^{B \times n_{d}} \\
    a[i] &\in \mathbb{R}^{B \times n_{a}}
  \end{split}
\end{align}
\end{document}

This leaves a lot of blank space on the page, which you might care more about if it’s for a printed textbook. On the bright side, paper is an excellent carbon sink!

You can tweak this to your taste, such as changing the horizontal spacing of a line or using \begin{multilined} instead, putting only the list of variable definitions in a nested aligned environment, etc.

If you don’t like “where” on its own line, you could replace \shortintertext{where} with \textnormal{ where }.

Answer 2

This is somewhat opinion based, but I'll say how I'd treat the business anyway, because typographical and mathematical features are involved.

To begin with, the equation number is only pertinent to the equation and not to the explanatory clauses. Also, this seems to be higher level mathematics (possibly a research paper), so I'd start with assuming that my readers do know what I'm talking about. For instance, at this point they will already know what B, n_d and n_a mean.

You just need to specify something about the symbols in the equation, so just add the explanations after the equation. I always start from the principle that readers can read and that a mathematical text, particularly in a research paper, should be as plain as possible.

In other words, the explanatory clauses need no special formatting: when readers have seen the equation, they'll continue reading on and find the clauses.

\documentclass[twocolumn]{article}
\usepackage{amsmath}
\usepackage{newtxtext,newtxmath}

\usepackage{lipsum} % for mock text

\begin{document}

\lipsum[1][1-3]

We can state our main equation
\begin{equation}
\bigl[d[i], a[i]\bigr] = f_{i}(M[i]\cdot f)
\end{equation}
where $d[i] \in \mathbb{R}^{B \times n_{d}}$ and $a[i] \in \mathbb{R}^{B \times n_{a}}$.
After the $n$th step two outputs are produced.
\lipsum

\end{document}

Please, don't do $n^{th}$. It's wrong. If you really want a superscripted “th”, use $n$\textsuperscript{th}. But $n$th or $n$-th is better.