One reason for numbering things like figures and equations is to refer the reader to them, as in "See Figure 3 for more details."
One can generate numbered references to anything for which LaTeX maintains a counter by placing a \label command with some mnemonic name in the appropriate environment. The reference is then generated by use of the \ref command with the same mnemonic.
For example
\begin{equation}
\label{eqno1}
c \geq \sqrt{a^2+b^2+1}
\end{equation}
.... intervening text ...
From Eq.~(\ref{eqno1}) we see ...
Assuming that this is the first equation, the last line will produce "From Eq. (1) we see ..." Note the use of the ~ to ensure that there is no line break between "Eq." and "(1)." Also note that the \ref command produces only the number; if you want the number to be in parentheses (as shown here) you have to include them in your text.
Bibliographic citations work somewhat differently. The following should be consulted:
See also
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