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I would like two of my lemmas to be numbered additionally by letters, e.g. Lemma 2a. and Lemma 2b, while the all other lemmas (and theorems etc.) are numbered normally by numbers.

\newtheorem{lemma}{Lemma}
\begin{document}
\begin{lemma}
This should be Lemma 1.
\end{lemma}
\begin{lemma}
This should be Lemma 2a.
\end{lemma}
\begin{lemma}
This should be Lemma 2b.
\end{lemma}
\begin{lemma}
This should be Lemma 3.
\end{lemma}
\end{document}

Duplicate, I guess: tex.stackexchange.com/q/43346/4427

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