Definition:Fractional Part

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Let $x \in \R$ be a real number.

Let $\floor x$ be the floor function of $x$.

The fractional part of $x$ is the difference:

$\fractpart x := x - \floor x$

Beware, of course, not to get $\fractpart x$ confused with the singleton set containing $x$.

Also known as

Some sources, particularly those aimed for the grade-school and muggle market, refer to this as the decimal part, or (even worse) just the decimal.

This misnomer arises from the fact that it is the part of the number after the decimal point.

Burn it with fire.

Also see


$x \bmod 1 = \fractpart x$