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The **fractional part** or **decimal part**^{[1]} of a non‐negative real number is the excess beyond that number's integer part. The latter is defined as the largest integer not greater than x, called *floor* of x or . Then, the fractional part can be formulated as a difference:

- .

For a positive number written in a conventional positional numeral system (such as binary or decimal), its fractional part hence corresponds to the digits appearing after the radix point. The result is a real number in the half-open interval [0, 1).

However, in case of negative numbers, there are various conflicting ways to extend the fractional part function to them: It is either defined in the same way as for positive numbers, i.e., by (Graham, Knuth & Patashnik 1992),^{[2]} or as the part of the number to the right of the radix point (Daintith 2004),^{[3]} or by the odd function:^{[4]}

with as the smallest integer not less than x, also called the ceiling of x. By consequence, we may get, for example, three different values for the fractional part of just one x: let it be −1.3, its fractional part will be 0.7 according to the first definition, 0.3 according to the second definition, and −0.3 according to the third definition, whose result can also be obtained in a straightforward way by

- .

The and the "odd function" definitions permit for unique decomposition of any real number x to the sum of its integer and fractional parts, where "integer part" refers to or respectively. These two definitions of fractional-part function also provide idempotence.

The fractional part defined via difference from ⌊ ⌋ is usually denoted by curly braces:

Every real number can be essentially uniquely represented as a continued fraction, namely as the sum of its integer part and the reciprocal of its fractional part which is written as the sum of *its* integer part and the reciprocal of *its* fractional part, and so on.

**^**"Decimal part". Oxford Dictionaries. Archived from the original on February 15, 2018. Retrieved 2018-02-15.**^**Graham, Ronald L.; Knuth, Donald E.; Patashnik, Oren (1992),*Concrete mathematics: a foundation for computer science*, Addison-Wesley, p. 70, ISBN 0-201-14236-8**^**Daintith, John (2004),*A Dictionary of Computing*, Oxford University Press**^**Weisstein, Eric W. "Fractional Part." From MathWorld--A Wolfram Web Resource