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Colon character : & \colon

Synopsis, one of:

:
\colon

In mathematics, the colon character, :, is a relation.

With side ratios \( 3:4 \) and \( 4:5 \), the triangle is right.

Ordinary LaTeX defines \colon to produce the colon character with the spacing appropriate for punctuation, as in set-builder notation \{x\colon 0\leq x<1\}.

But the widely-used amsmath package defines \colon for use in the definition of functions f\colon D\to C. So if you want the colon character as a punctuation then use \mathpunct{:}.