Enumerating theorems with numbers and letters

Question

I want to have my theorem environments to enumerate using both numbers and letters. Right now I have this:

\documentclass{article}
\usepackage{amsthm}
\newtheorem{theorem}{Theorem}[section]
\begin{document}
\setcounter{section}{1}
\begin{theorem}
    This enumerates as ``1.1." but I want ``1.1.a."
\end{theorem}
\begin{theorem}
    Then this one would be ``1.1.b"
\end{theorem}
\end{document}

I know I can use

\renewcommand*{\thetheorem}{\alph{theorem}}

to enumerate with letters but I'm not sure how to proceed in applying that to this situation.

Answer 1

If you want to prefix the section and subsection numbers to the theorem "number" (here: lowercase letter), you need to replace \newtheorem{theorem}{Theorem}[section] with \newtheorem{theorem}{Theorem}[subsection].

\documentclass{article}
\usepackage{amsthm} % for '\newtheorem' macro
\newtheorem{theorem}{Theorem}[subsection]
\renewcommand\thetheorem{\thesubsection.\alph{theorem}}

\begin{document}
\setcounter{section}{1}
\setcounter{subsection}{1}

\begin{theorem}
    I want ``1.1.a.'' -- check!
\end{theorem}
\begin{theorem}
    This one should be ``1.1.b.'' -- check!
\end{theorem}
\end{document}