Annoying space before the first item in an enumeration which starts a theorem

Question

Consider the following example:

\documentclass{article}
\newtheorem{Theorem}{Theorem}
\begin{document}
\begin{Theorem}
On the other hand, this one has only one claim.
\end{Theorem}
\begin{Theorem}
\begin{enumerate}
\item First claim
\item Second claim
\end{enumerate}
\end{Theorem}
\end{document}

Typesetting it results in:

Notice there is extra horizontal space before the first item in the list of the second theorem.

Is there a sensible way to get rid of that?

Ideally, the solution would work with amsthm, thmtools, enumitem.

N.B.: this is related to Is it possible to skip the first line in a theorem environment? but not quite the same.

Answer 1

Two variants of a solution using the enumitem package. In the first case, two enumerate environments were used; the first one uses leftamargin=* to suppress the undesired horizontal space; the second enumerate uses resume to keep the numbering but preserves the standard value for \leftmargin; the vertical spacing between the two environments was also corrected. In the second case, the option leftmargin=* was also used, but this time only one enumerate environment was used so the horizontal space will be removed for all the items.

\documentclass{article}
\usepackage{enumitem}

\newtheorem{Theorem}{Theorem}

\begin{document}

\begin{Theorem}
   On the other hand, this one has only one claim.
\end{Theorem}

\begin{Theorem}
  \begin{enumerate}[leftmargin=*]
    \item First claim
  \end{enumerate}\vspace*{-2\partopsep}
  \begin{enumerate}[resume]
    \item Second claim
  \end{enumerate}
\end{Theorem}

\begin{Theorem}
   On the other hand, this one has only one claim.
\end{Theorem}

\begin{Theorem}
  \begin{enumerate}[leftmargin=*]
    \item First claim
    \item Second claim
  \end{enumerate}
\end{Theorem}

\end{document}

The result:

Answer 2

You can use a \@labels trick (as in Phil Hirschhorn's answer to “Avoiding a line break at the beginning of an enumerate”). Here's the result:

There are two ways you can use this. The first is by manually putting a \thmenumhspace{-1em} before each \begin{enumerate} inside a theorem:

\documentclass{article}
%\usepackage{amsthm,enumitem,thmtools}% works with or without
\newtheorem{Theorem}{Theorem}
\makeatletter
\newcommand{\thmenumhspace}[1]{\sbox{\@labels}{\unhbox\@labels\hskip#1}}
\makeatother
\begin{document}
\begin{Theorem}
On the other hand, this one has only one claim.
\end{Theorem}
\begin{Theorem}\thmenumhspace{-1em}
\begin{enumerate}
\item First claim
\item Second claim
\end{enumerate}
\end{Theorem}
\end{document}

With a little more work, this can be automated, hopefully without any side effects (this time, you need amsthm for it to work correctly):

\documentclass{article}
\usepackage{amsthm,enumitem,thmtools}% needs amsthm
\newtheorem{Theorem}{Theorem}
\makeatletter
\newcommand{\thmenumhspace}[1]{\sbox{\@labels}{\unhbox\@labels\hskip#1}}
\let\original@item\item
\newcommand{\RedefineItem}{%
  \def\item{\let\item\original@item\thmenumhspace{-1em}\original@item}
}
\let\original@Theorem\Theorem
\def\Theorem{
   \@ifnextchar[{\Theorem@Opt}
                {\Theorem@NoOpt}
}
\def\Theorem@Opt[#1]{%
   \@ifnextchar\begin{\RedefineItem\original@Theorem[#1]}%
                     {\original@Theorem[#1]}%
}
\def\Theorem@NoOpt{%
   \@ifnextchar\begin{\RedefineItem\original@Theorem}%
                     {\original@Theorem}%
}
\makeatother
\begin{document}
\begin{Theorem}
On the other hand, this one has only one claim.
\end{Theorem}
\begin{Theorem}
\begin{enumerate}
\item First claim
\item Second claim
\end{enumerate}
\end{Theorem}
\begin{Theorem}
On the other hand, this one has only one claim.
\begin{enumerate}
\item First claim
\item Second claim
\end{enumerate}
\end{Theorem}
\begin{Theorem}[Title]
\begin{enumerate}
\item First claim
\item Second claim
\end{enumerate}
\end{Theorem}
\begin{Theorem}[Title]
On the other hand, this one has only one claim.
\begin{enumerate}
\item First claim
\item Second claim
\end{enumerate}
\end{Theorem}
\end{document}

Answer 3

I found this trick that might help:

\documentclass{article}
\newtheorem{Theorem}{Theorem}
\begin{document}
\begin{Theorem}
On the other hand, this one has only one claim.
\end{Theorem}
\begin{Theorem}
\mbox{}
\begin{itemize}    
\item[(i)] item one.    
\item[(ii)] item two.    
\item[(iii)]  item three.    
\end{itemize}
\end{Theorem}
\end{document}

Answer 4

You can hack a bit the first \item as follows:

\documentclass{article}
\newtheorem{Theorem}{Theorem}
\newcommand*\fixitem {\item[]%
  \refstepcounter{enumi}\hskip-\leftmargin\labelenumi\hskip\labelsep}
\begin{document}
\begin{Theorem}
On the other hand, this one has only one claim.
\end{Theorem}
\begin{Theorem}
\begin{enumerate}
\fixitem First claim
\item Second claim
\end{enumerate}
\end{Theorem}
\end{document}

and the result is: