How to have non-italic text from \cite[]{} within theorem using amsthm and article class?

Question

I have a tex file:

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

\numberwithin{equation}{section}
\newtheorem{theorem}{Theorem}[section]
\theoremstyle{definition}
\newtheorem{assumption}[theorem]{Assumption}

\newcommand{\RR}{\mathbb{R}}
\newcommand{\TT}{\mathbb{T}}
\setlist{font=\normalfont}

\begin{document}

\section{Introduction}

\begin{assumption} \label{main_assmpt1}
There exist constants $C>0, \alpha \in (0, 1), \beta \in (0, 1)$ such that for $t \in \TT$ and $x, y \in \RR^d$:
\begin{enumerate}[label=(A\arabic*)]
\item $\nu$ has a density $\ell_\nu \in C^\alpha_b (\RR^d)$.
\item $a_t$ is invertible.
\end{enumerate}
\end{assumption}

\begin{theorem}
Let \cref{main_assmpt1} hold.
\begin{enumerate}
\item \cite[Theorem 2.1]{mckean1966class} ...
\end{enumerate}
\end{theorem}

\begin{thebibliography}{10}
\bibitem{mckean1966class}
Henry~P McKean~Jr.
\newblock A class of markov processes associated with nonlinear parabolic
  equations.
\newblock {\em Proceedings of the National Academy of Sciences},
  56(6):1907--1911, 1966.
\end{thebibliography}
\end{document}

I particularly like the non-italic quoted theorem name:

When I change from \documentclass[11pt]{amsart} to \documentclass[11pt]{article}, I lost this desired appearance:

Could you explain how to keep this aesthetic feature within the class article?

Answer 1

amsart uses:

\def\@citestyle{\m@th\upshape\mdseries}
\let\citeform\@firstofone
\def\@cite#1#2{{%
  \@citestyle[\citeform{#1}\if@tempswa, #2\fi]}}

to explicitly set cites \upshape. The default LaTeX cite command does not do something like this. The default \@cite definition of LaTeX is:

\def\@cite#1#2{[{#1\if@tempswa , #2\fi}]}

So inside a theorem the theorem body font is used for the cite, too. But you can redefine it similar, by coping the amsart code:

\documentclass[11pt]{article}
\usepackage{amsthm}
\usepackage{amssymb,enumitem,mathtools,fullpage,microtype}
\usepackage[hypertexnames=false]{hyperref}
\hypersetup{
  colorlinks=true,
  linkcolor=blue,
  filecolor=blue,
  urlcolor=red,
  citecolor=magenta
}
\usepackage[noabbrev,capitalize,nameinlink]{cleveref}
\usepackage{autonum}
\newcommand{\RR}{\mathbb{R}}
\newcommand{\TT}{\mathbb{T}}
\setlist{font=\normalfont}

\numberwithin{equation}{section}
\newtheorem{theorem}{Theorem}[section]
\theoremstyle{definition}
\newtheorem{assumption}[theorem]{Assumption}

\makeatletter
\def\@citestyle{\m@th\upshape\mdseries}
\let\citeform\@firstofone
\def\@cite#1#2{{%
  \@citestyle[\citeform{#1}\if@tempswa, #2\fi]}}
\makeatother

\begin{document}

\section{Introduction}

\begin{assumption} \label{main_assmpt1}
There exist constants $C>0, \alpha \in (0, 1), \beta \in (0, 1)$ such that for $t \in \TT$ and $x, y \in \RR^d$:
\begin{enumerate}[label=(A\arabic*)]
\item $\nu$ has a density $\ell_\nu \in C^\alpha_b (\RR^d)$.
\item $a_t$ is invertible.
\end{enumerate}
\end{assumption}

\begin{theorem}
Let \cref{main_assmpt1} hold.
\begin{enumerate}
\item \cite[Theorem 2.1]{mckean1966class} ...
\end{enumerate}
\end{theorem}

\begin{thebibliography}{10}
\bibitem{mckean1966class}
Henry~P McKean~Jr.
\newblock A class of markov processes associated with nonlinear parabolic
  equations.
\newblock {\em Proceedings of the National Academy of Sciences},
56(6):1907--1911, 1966.
\end{thebibliography}
\end{document}

Notes:

• The code above works if you really make your bibliography manually. If you use a package like biblatex, please see the manual of that package for more information about formatting of cites. And yes, I would recommend to use such a package, i.e., biblatex.

• If want not only the cites but also the rest of the theorem body to not use an italic shape, you should use either \newtheoremstyle to define a corresponding style, or use, e.g., \theoremstyle{definition}.

• If you, e.g., use natbib you can instead use something like \bibpunct{\upshape\mdseries[}{]}{,}{n}{}{,}. See the natbib manual for more information about \bibpunct.