The numbering of an equation does not show up

Question

I have a tex file

%
\documentclass[11pt]{article}
\usepackage{mathtools,amssymb,amsthm,enumitem,tcolorbox,fullpage,microtype,multirow}
\usepackage{showkeys}
\usepackage[utf8]{inputenc}
\usepackage[T1]{fontenc}
\usepackage[mathscr]{eucal}
\usepackage[hypertexnames=false]{hyperref}
\hypersetup{
  colorlinks=true,
  linkcolor=blue,
  filecolor=blue,
  urlcolor=red,
  citecolor=magenta
}
\usepackage[noabbrev,capitalize,nameinlink]{cleveref}
\usepackage{autonum}
\setlist{font=\normalfont}

\DeclareMathOperator*{\supp}{supp}
\newcommand{\RR}{\mathbb{R}}
\newcommand{\TT}{\mathbb{T}}
\newcommand{\NN}{\mathbb{N}}
\newcommand{\PP}{\mathbb{P}}
\newcommand{\EE}{\mathbb{E}}
\newcommand{\FF}{\mathbb{F}}
\newcommand{\diff}{\, \mathrm d}

\begin{document}

\section{Introduction}
We fix $T>0$ and let $\TT$ be the interval $[0, T]$. We consider measurable functions
\begin{align}
b &: \TT \times {\RR}^d \to {\RR}^d, \\
\sigma &: \TT \times  {\RR}^d \to {\RR}^d \otimes {\RR}^m.
\end{align}

Let $(B_t, t \ge 0)$ be a $m$-dimensional Brownian motion and $\FF := (\mathcal F_t, t \ge 0)$ an admissible filtration on a probability space $(\Omega, \mathcal A, \PP)$. We consider the SDE
\begin{align} \label{main_eq1}
\diff X_t = b(t, X_t) \diff t + \sigma (t, X_t) \diff B_t,
\quad t \in \TT,
\end{align}
where the distribution of $X_0$ is $\nu$. Let $a := \sigma \sigma^\top$. For simplicity, we denote $b_t (x) := b(t, x), \sigma_t(x) := \sigma (t, x)$ and $a_t(x) := a (t, x)$.

Equation \eqref{main_eq1} is weakly well-posed.

\end{document}

which is compiled without error. However, the numbering of the equation does not show up:

Could you explain the reason behind this problem and how to solve it?

Answer 1

The culprit is the combination of showkeys and autonum, which don't go along together.

\documentclass[11pt]{article}
\usepackage{mathtools,amssymb,amsthm,enumitem,tcolorbox,fullpage,microtype,multirow}
\usepackage[T1]{fontenc}
\usepackage[mathscr]{eucal}
\usepackage[hypertexnames=false]{hyperref}
\hypersetup{
  colorlinks=true,
  linkcolor=blue,
  filecolor=blue,
  urlcolor=red,
  citecolor=magenta
}
\usepackage[noabbrev,capitalize,nameinlink]{cleveref}
\usepackage{autonum}

\setlist{font=\normalfont}

\DeclareMathOperator*{\supp}{supp}
\newcommand{\RR}{\mathbb{R}}
\newcommand{\TT}{\mathbb{T}}
\newcommand{\NN}{\mathbb{N}}
\newcommand{\PP}{\mathbb{P}}
\newcommand{\EE}{\mathbb{E}}
\newcommand{\FF}{\mathbb{F}}
\newcommand{\diff}{\mathop{}\!\mathrm{d}}% sigh!

\begin{document}

\section{Introduction}
We fix $T>0$ and let $\TT$ be the interval $[0, T]$. We consider measurable functions
\begin{align}
b &: \TT \times \RR^d \to \RR^d, \\
\sigma &: \TT \times  \RR^d \to \RR^d \otimes \RR^m.
\end{align}

Let $(B_t, t \ge 0)$ be a $m$-dimensional Brownian motion and 
$\FF := (\mathcal F_t, t \ge 0)$ an admissible filtration on 
a probability space $(\Omega, \mathcal A, \PP)$. We consider the SDE
\begin{equation}\label{main_eq1}
\diff X_t = b(t, X_t) \diff t + \sigma (t, X_t) \diff B_t,
\quad t \in \TT,
\end{equation}
where the distribution of $X_0$ is $\nu$. Let $a := \sigma \sigma^\top$. 
For simplicity, we denote $b_t (x) := b(t, x), \sigma_t(x) := \sigma (t, x)$ 
and $a_t(x) := a (t, x)$.

Equation \eqref{main_eq1} is weakly well-posed.

\end{document}

Note that {\RR}^d has superfluous braces and that your definition of \diff isn't correct (and I'm not referring to the abominable upright d, but to the forced space).

Don't use align for single equations.

What can you do? You can use showkeys while writing the document; cross references won't work, but you don't need them in that phase. Remove the package when ready for the production version.