How to avoid newline between enumeration and theorem?

Question

When I insert an theorem/definition environment at the start of an item of enumerate, it creates a newline automatically before Theorem. Is it somehow possible to remove this enforced newline so that the enumeration item (e.g. (a) ) is on the same line as the theorem beginning?

The following is a minimal example:

\documentclass{article}
\usepackage{amsthm}
\usepackage{enumerate}
\newtheorem{theorem}{Theorem}
\begin{document}

\begin{enumerate}[(a)]
  \item % avoid newline here
    \begin{theorem} Some statement. \end{theorem}
\end{enumerate}

\end{document}

Answer 1

Perhaps the easiest way is to define another environment for theorems which should not start with a line break. You can tweak the appearance if you want.

\documentclass{article}
\usepackage{amsthm}
\usepackage{enumerate}
\newtheorem{theorem}{Theorem}

% For theorems which do not start with a line break
\makeatletter
\newenvironment{nonewlinetheorem}
{\refstepcounter{theorem}\textbf{Theorem~\thetheorem.\space}\itshape}{\par}
\makeatother

\begin{document}

\begin{enumerate}[(a)]
\item \begin{theorem} Some statement. \end{theorem}
\item
  \begin{nonewlinetheorem}
    Another theorem statement.
  \end{nonewlinetheorem}

\end{enumerate}

\end{document}

Answer 2

It's a little bit of a hack, but negative \vspace is a fairly common solution in LaTeX and will allow you to use this solution on any Theorem/Proposition/Lemma etc. environment you like without defining a new one for the purpose. As an example:

\documentclass{article}
\usepackage{amsthm}
\usepackage{enumerate}
\newtheorem{lemma}{Lemma}
\newtheorem{theorem}{Theorem}

\begin{document}

\begin{enumerate}[(a)]
\item \leavevmode \vspace{-2em}
    \begin{lemma} $e^{i\theta}\equiv\cos(\theta)+i\sin(\theta).$ \end{lemma}
\item \leavevmode \vspace{-2em}
    \begin{theorem} $e^{i\pi}=-1$. \end{theorem}
\end{enumerate}

\end{document}

gives the following:

Here \leavevmode is necessary to enter horizontal mode, otherwise \vspace produces a \vskip which ends the current paragraph, which does not then produce the desired affect.