# How to enumerate theorems with A.1, A.2 and so on

I'm currently using

\newtheorem{lemma}[abc]{Lemma A.}


but the spacing is off. abc is just a counter here. Can I instead have the A. be part of the enumeration? Or is there a way to remove the space after "Lemma A."? The enumeration should just be A.1, A.2, A.3 and so on, the letter A never changes.

Redefine the abc counter representation to always be preceded by A.:

\documentclass{article}

\newcounter{abc}
\newtheorem{lemma}[abc]{Lemma}
\renewcommand{\theabc}{A.\arabic{abc}}

\begin{document}

Also see Lemma~\ref{lem:third}.
\begin{lemma}
First lemma.
\end{lemma}

\begin{lemma}[second]
Second lemma.
\end{lemma}

\begin{lemma}[last]
Third lemma.\label{lem:third}
\end{lemma}

\end{document}