The set obtained by adjoining two improper elements to the set of real numbers is normally called the set of (affinely) extended real numbers. Although the notation for this set is not completely standardized, is commonly used. The set may also be written in interval notation as . With an appropriate topology, is the twopoint compactification (or affine closure) of . The improper elements, the affine infinities and , correspond to ideal points of the number line. Note that these improper elements are not real numbers, and that this system of extended real numbers is not a field.
Instead of writing , many authors write simply . However, the compound symbol will be used here to represent the positive improper element of , allowing the individual symbol to be used unambiguously to represent the unsigned improper element of , the onepoint compactification (or projective closure) of .
A very important property of , which lacks, is that every subset of has an infimum (greatest lower bound) and a supremum (least upper bound). In particular, and, if is unbounded above, then . Similarly, and, if is unbounded below, then .
Order relations can be extended from to , and arithmetic operations can be partially extended. For ,
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However, the expressions , , and are undefined.
The above statements which define results of arithmetic operations on may be considered as abbreviations of statements about determinate limit forms. For example, may be considered as an abbreviation for "If increases without bound, then decreases without bound." Most descriptions of also make a statement concerning the products of the improper elements and 0, but there is no consensus as to what that statement should be. Some authors (e.g., Kolmogorov 1995, p. 193) state that, like and , and should be undefined, presumably because of the indeterminate status of the corresponding limit forms. Other authors (such as McShane 1983, p. 2) accept , at least as a convention which is useful in certain contexts.
Many results for other operations and functions can be obtained by considering determinate limit forms. For example, a partial extension of the function can be obtained for as
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The functions and can be fully extended to , with
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Some other important functions (e.g., and ) can be extended to , while others (e.g., , ) cannot. Evaluations of expressions involving and , derived by considering determinate limit forms, are routinely used by computer algebra languages such as the Wolfram Language when performing simplifications.
Floatingpoint arithmetic, with its two signed infinities, is intended to approximate arithmetic on (Goldberg 1991, pp. 2122).