Difference in theorem spacing between article and amsart

Question

I defined a new theorem environment style, which I called "slanted." When I use the default "article" style, then everything looks great. However, whenever I use "amsart", the new theorem style is not separated by any space from above or below. Does anyone know why that is the case? Below you can see both pieces of code and their outputs (for comparison, I also included a definition style environment, which in both cases looks fine)

\documentclass[12pt]{article}
\usepackage[utf8]{inputenc}
\usepackage{amsmath}
\usepackage{amssymb}
\usepackage{amsthm}
\usepackage{times,newtxmath}

\newtheoremstyle{slanted}
  {\topsep}%   Space above
  {\topsep}%   Space below
  {\slshape}%  Body font
  {}%          Indent amount (empty = no indent, \parindent = para indent)
  {\bfseries}% Thm head font
  {.}%         Punctuation after thm head
  {0.5em}%     Space after thm head: " " = normal interword space;
     %         \newline = linebreak
  {}%          Thm head spec (can be left empty, meaning `normal')

\theoremstyle{slanted}
\newtheorem{theorem}{Theorem}

\theoremstyle{definition}
\newtheorem*{de}{Definition}

\begin{document}

Mathematicians resolve the truth or falsity of conjectures by mathematical proof.
\begin{theorem}
Mathematicians seek out patterns.
\end{theorem}

   Mathematicians resolve the truth or falsity of conjectures by mathematical proof.
\begin{de}
Mathematicians seek out patterns.
\end{de}

Mathematicians resolve the truth or falsity of conjectures by mathematical proof.

\end{document}

The output of this code looks as follows:

Whereas if I use "amsart" instead of "article"

\documentclass[12pt]{amsart}
\usepackage[utf8]{inputenc}
\usepackage{amsmath}
\usepackage{amssymb}
\usepackage{amsthm}
\usepackage{times,newtxmath}

\newtheoremstyle{slanted}
  {\topsep}%   Space above
  {\topsep}%   Space below
  {\slshape}%  Body font
  {}%          Indent amount (empty = no indent, \parindent = para indent)
  {\bfseries}% Thm head font
  {.}%         Punctuation after thm head
  {0.5em}%     Space after thm head: " " = normal interword space;
     %         \newline = linebreak
  {}%          Thm head spec (can be left empty, meaning `normal')

\theoremstyle{slanted}
\newtheorem{theorem}{Theorem}

\theoremstyle{definition}
\newtheorem*{de}{Definition}

\begin{document}

Mathematicians resolve the truth or falsity of conjectures by mathematical proof.
\begin{theorem}
Mathematicians seek out patterns.
\end{theorem}

   Mathematicians resolve the truth or falsity of conjectures by mathematical proof.
\begin{de}
Mathematicians seek out patterns.
\end{de}

Mathematicians resolve the truth or falsity of conjectures by mathematical proof.

\end{document}

I get the following:

Answer 1

If you replace the "Thm head font" line by

   {\bfseries\showthe\topsep}% Thm head font

then with article class it reports in the .log file:

> 10.0pt plus 4.0pt minus 6.0pt.

whereas amsart gives

> 2.44998pt plus 2.44998pt.

This shows that the two document classes have made different decisions about the value of \topsep. Instead you can replace your style definition by something like:

\newtheoremstyle{slanted}
  {10pt plus 4pt minus 4pt}%   Space above
  {10pt plus 4pt minus 4pt}%   Space below
  {\slshape}%  Body font
  {}%          Indent amount (empty = no indent, \parindent = para indent)
  {\bfseries}% Thm head font
  {.}%         Punctuation after thm head
  {0.5em}%     Space after thm head: " " = normal interword space;
     %         \newline = linebreak
  {}%          Thm head spec (can be left empty, meaning `normal')

so the separation is specified explicitly.

In amsart, the \topsep is set to \smallskipamount with is a quarter of \bigskipamount = .7\baselineskip plus .7\baselineskip.

For article.cls with the 12pt option, the value is set in size12.clo under the setting for \@listi.