LaTeX numbering

One advantage of LaTeX over the other TeX-flavors is that it provides an automatic numbering of the sections, theorems, equations etc., together with an easy way to refer to these numbers.

The value of a counter can be changed with a command of the type

    
      \setcounter{equation}{0}

One can achive the reseting of the equation counter at the beginning of each section and the inclusion of the section number in the equation numbering with the command

    
      \numberwithin{equation}{section}

To add a label to any of the items which received a number, use

    
      \label{label-name}

One can refer anywhere else in the text to it with one of the following:

    
      \ref{label-name}
      \eqref{label-name}
      \pageref{label-name}

The meaning is the following:

\ref gives the "number" to which the label refers,
\eqref includes the "number" in parantheses,
\pageref gives the page where the reference was created.

One can adjust the way items are numbered by re-defining some of the commands (the first example is essentially equivalent to \numberwithin{equation}{section}):

    
      \renewcommand{\theequation}{\thesection.\arabic{equation}}
      \renewcommand{\thesection}{\arabic{section}}
      \renewcommand{\thesubsection}{(\alph{subsection})}

The \newtheorem command defines a new "theorem-like" environment. Its syntax is described below. Two arguments are required (as in the first form), the others are optional.

    
      \newtheorem{env-name}{label-text}
      \newtheorem{env-name}[other_env]{label-text}
      \newtheorem{env-name}[other_env]{label-text}[within]

where:

env-name is the name of the new environment
label-text is the label printed (usually in bold) at the beginning (e.g., Theorem, Lemma, etc.)
other_env is the environment whose numbering should be used by the env-name (e.g., you might want theorems and lemmas to be numbered in the same sequence)
within is a counter (e.g., section); it indicates that the numbering should include this counter (and start from 1 each time this counter changes)

Below is an example of macros that define the way theorems etc. are numbered and presented.
With these definitions numbering includes the section number, starts from 1 in each section, and theorems, lemmas, etc. are numbered in the same sequence. [IMHO, this is highly desirable, otherwise the reader might have a hard time to find Proposition 2.1 as it could be after Definition 2.5.]
E.g., Section 2 will have Theorem 2.1, Theorem 2.2, Remark 2.3, Definition 2.4, Remark 2.5, Lemma 2.6 and so on.

    
      \newtheorem{thm}{Theorem}[section]
      \newtheorem{lemma}[thm]{Lemma}
      \newtheorem{prop}[thm]{Proposition}
      \newtheorem{cor}[thm]{Corollary}
      \newtheorem{defn}[thm]{Definition}
      \newtheorem{examp}[thm]{Example}
      \newtheorem{conj}[thm]{Conjecture}
      \newtheorem{rmk}[thm]{Remark}

NOTE that one can also use some of these environments without a number as well:

    
      \newtheorem*{thm*}{Theorem}

which is invoked with

    
      \begin{thm*}
      TEXT (formatted as in a Theorem)
      \end{thm*}

For more on variations of the fonts used, see this part in the Fancy Stuff Section.