Footnote marker placement in heading of theorem/definition/etc.

Question

I have a definition, and a footnote which refers to the whole definition.  So I’d ideally like an output looking something like:

Definition 1.5³.   A footnote is a small aside, usually placed at the bottom of a page and marked by a tag in the main text.

---

³Some people think that footnotes are bad style.

Is there a good way to get this placement of the footnote marker?  Neither of the following comes out right (for obvious reasons):

\begin{definition}\footnote{Footnote text.} We define \ldots

\begin{definition}[\footnote{Footnote text.}] We define \ldots

I’m using amsthm, but answers for without that would be interesting too.

Answer 1

Change the definition of \thetheorem inside a group so as to include the desired footnote. (I use the \apptocmd command of the etoolbox package to amend the definition without looking up the particulars of the original.)

\documentclass{article}

\usepackage{amsthm}
\newtheorem{theorem}{Theorem}

\usepackage{etoolbox}

\begin{document}

\begingroup
\apptocmd{\thetheorem}{\protect\footnote{A footnote.}}{}{}

\begin{theorem}
Some text.
\end{theorem}

\endgroup

\begin{theorem}
Some text.
\end{theorem}

\end{document}

EDIT: Interestingly, this only seems to work with amsthm.

Answer 2

I don't have enough reputation to add a comment to the previous answer, so let me add the following as an answer instead.

I found what Philippe Goutet said in the comment to lockstep's answer to be true, namely that a footnote appears in references to the theorem too. Perhaps it is obvious to others how to implement Philippe's fix, but it wasn't to me: it seems that one also needs to add something like \makeatletter ... \makeatother somewhere. The following version of lockstep's answer thus works for me.

\documentclass{article}

\usepackage{amsthm}
\newtheorem{theorem}{Theorem}

\usepackage{etoolbox}

\begin{document}

\begingroup
\makeatletter
\apptocmd{\thetheorem}{\unless\ifx\protect\@unexpandable@protect\protect\footnote{A footnote.}\fi}{}{}
\makeatother

\begin{theorem}\label{mainthm}
Some text.
\end{theorem}

\endgroup

Compare the following to Theorem \ref{mainthm}.

\begin{theorem}
Some text.
\end{theorem}

\end{document}

The result:

I don't know if there is some better place to insert the commands, but the above seems to work.