Theorem numbering in LaTeX

Question

I have a problem with theorem numbering in LaTeX. I can make it number by subsection, e.g

Theorem 1.2.1

for the first theorem in the second subsection of the first section. But I need it to show me only the numbers of the subsection and the theorem, but not the section number, like this:

Theorem 2.1

I use

\newtheorem{thm}{Theorem}[subsection]

for the numbering.

Answer 1

Does this work?

\newtheorem{thm}{Theorem}[section]

See these LaTeX tips.

Answer 2

Insert this line in your preamble (or anywhere else before the \newtheorem statement):

\renewcommand{\thesubsection}{\arabic{subsection}}

This will reset the numbering command of the thm environment to ignore the section numbers (when numbering theorems) and display only the subsection numbers and theorem numbers. Section numbers will still be displayed in front of section headings, just not the theorems included within the sections. So, just as you describe, the first theorem in the second subsection of the first section will be numbered 2.1. Alternatives to \arabic include:

• \Roman - produces capital roman numbers, such as II.1
• \roman - produces lower-case roman numbers, such as ii.1
• \Alph - produces capital letters, such as B.1
• \alph - produces lower-case letters, such as b.1

Answer 3

There's no easy way to do this. The AMS Theorem Package only provides a way to control when numbering resets (section, subsection), if it's tied to other environments (corollary, lemma) and number order ("1.1 Theorem" vs. "Theorem 1.1").

Theorem's get their numbering from the \thesection or \thesubsection command. You can redefine the \thesubsection command to get the numbering you want, but that will also affect everything else that uses \thesubsection.