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In mathematics, the affinely extended real number system is obtained from the real number system \R by adding two infinity elements: +\infty and where the infinities are treated as actual numbers. It is useful in describing the algebra on infinities and the various limiting behaviors in calculus and mathematical analysis, especially in the theory of measure and integration. The affinely extended real number system is denoted \overline or \infty, +\infty/math> or When the meaning is clear from context, the symbol +\infty is often written simply as

Motivation



Limits

It is often useful to describe the behavior of a function as either the argument x or the function value f(x) gets "infinitely large" in some sense. For example, consider the function :f(x) = \frac. The graph of this function has a horizontal asymptote at Geometrically, when moving increasingly farther to the right along the x-axis, the value of / approaches 0. This limiting behavior is similar to the limit of a function \lim_ f(x) in which the real number x approaches except that there is no real number to which x approaches. By adjoining the elements +\infty and -\infty to it enables a formulation of a "limit at infinity", with topological properties similar to those for To make things completely formal, the Cauchy sequences definition of \R allows defining +\infty as the set of all sequences \left\ of rational numbers, such that every M \in \R is associated with a corresponding N \in \N for which a_n > M for all The definition of -\infty can be constructed similarly.

Measure and integration

In measure theory, it is often useful to allow sets that have infinite measure and integrals whose value may be infinite. Such measures arise naturally out of calculus. For example, in assigning a measure to \R that agrees with the usual length of intervals, this measure must be larger than any finite real number. Also, when considering improper integrals, such as :\int_1^\frac the value "infinity" arises. Finally, it is often useful to consider the limit of a sequence of functions, such as :f_n(x) = \begin 2n\left(1-nx\right), & \mbox 0 \leq x \leq \frac \\ 0, & \mbox \frac < x \leq 1 \end Without allowing functions to take on infinite values, such essential results as the monotone convergence theorem and the dominated convergence theorem would not make sense.

Order and topological properties

The affinely extended real number system can be turned into a totally ordered set, by defining -\infty \leq a \leq +\infty for all With this order topology, \overline has the desirable property of compactness: every subset of \overline\R has a supremum and an infimum (the infimum of the empty set is +\infty, and its supremum is -\infty). Moreover, with this topology, \overline\R is homeomorphic to the unit interval Thus the topology is metrizable, corresponding (for a given homeomorphism) to the ordinary metric on this interval. There is no metric that is an extension of the ordinary metric on In this topology, a set U is a neighborhood of if and only if it contains a set \ for some real number The notion of the neighborhood of -\infty can be defined similarly. Using this characterization of extended-real neighborhoods, the specially defined limits for x tending to +\infty and and the specially defined concepts of limits equal to +\infty and reduce to the general topological definition of limits.

Arithmetic operations

The arithmetic operations of \R can be partially extended to \overline\R as follows: : \begin a + \infty = +\infty + a & = +\infty, & a & \neq -\infty \\ a - \infty = -\infty + a & = -\infty, & a & \neq +\infty \\ a \cdot (\pm\infty) = \pm\infty \cdot a & = \pm\infty, & a & \in (0, +\infty] \\ a \cdot (\pm\infty) = \pm\infty \cdot a & = \mp\infty, & a & \in [-\infty, 0) \\ \frac & = 0, & a & \in \mathbb \\ \frac & = \pm\infty, & a & \in (0, +\infty) \\ \frac & = \mp\infty, & a & \in (-\infty, 0) \end For exponentiation, see . Here, means both and while means both and The expressions \infty - \infty, 0 \times (\pm\infty) and \pm\infty/\pm\infty (called [[indeterminate forms) are usually left [[Defined and undefined|undefined. These rules are modeled on the laws for [[Limit_of_a_function#Limits_involving_infinity|infinite limits. However, in the context of probability or measure theory, 0 \times \pm\infty is often defined as When dealing with both positive and negative extended real numbers, the expression 1/0 is usually left undefined, because, although it is true that for every real nonzero sequence f that converges to the reciprocal sequence 1/f is eventually contained in every neighborhood of it is ''not'' true that the sequence 1/f must itself converge to either -\infty or \infty. Said another way, if a continuous function f achieves a zero at a certain value then it need not be the case that 1/f tends to either -\infty or \infty in the limit as x tends to This is the case for the limits of the identity function f(x) = x when x tends to 0, and of f(x) = x^2 \sin \left( 1/x \right) (for the latter function, neither -\infty nor \infty is a limit of even if only positive values of x are considered). However, in contexts where only non-negative values are considered, it is often convenient to define For example, when working with power series, the radius of convergence of a power series with coefficients a_n is often defined as the reciprocal of the limit-supremum of the sequence Thus, if one allows 1/0 to take the value then one can use this formula regardless of whether the limit-supremum is 0 or not.

Algebraic properties

With these definitions, \overline\R is ''not'' even a semigroup, let alone a group, a ring or a field as in the case of However, it has several convenient properties: * a + (b + c) and (a + b) + c are either equal or both undefined. * a + b and b + a are either equal or both undefined. * a \cdot (b \cdot c) and (a \cdot b) \cdot c are either equal or both undefined. * a \cdot b and b \cdot a are either equal or both undefined * a \cdot (b + c) and (a \cdot b) + (a \cdot c) are equal if both are defined. * If a \leq b and if both a + c and b + c are defined, then * If a \leq b and c > 0 and if both a \cdot c and b \cdot c are defined, then In general, all laws of arithmetic are valid in \overline\R—as long as all occurring expressions are defined.

Miscellaneous

Several functions can be continuously extended to \overline\R by taking limits. For instance, one may define the extremal points of the following functions as follow:\exp(-\infty) = 0, \ \ln(0) = -\infty, \ \tanh(\pm\infty) = \pm 1, \ \arctan(\pm\infty) = \pm\frac Some singularities may additionally be removed. For example, the function 1/x^2 can be continuously extended to \overline\R (under ''some'' definitions of continuity), by setting the value to +\infty for and 0 for x = +\infty and On the other hand, the function 1/x can ''not'' be continuously extended, because the function approaches -\infty as x approaches 0 from below, and +\infty as x approaches 0 from above. A similar but different real-line system, the projectively extended real line, does not distinguish between +\infty and -\infty (i.e. infinity is unsigned). As a result, a function may have limit \infty on the projectively extended real line, while in the affinely extended real number system, only the absolute value of the function has a limit, e.g. in the case of the function 1/x at On the other hand :\lim_ and \lim_ correspond on the projectively extended real line to only a limit from the right and one from the left, respectively, with the full limit only existing when the two are equal. Thus, the functions e^x and \arctan(x) cannot be made continuous at x = \infty on the projectively extended real line.

See also

* Division by zero * Extended complex plane * Extended natural numbers * Improper integral * Infinity * Log semiring * Series (mathematics) * Projectively extended real line * Computer representations of extended real numbers, see and IEEE floating point

Notes




References




Further reading

* * {{Real numbers Category:Infinity Category:Real numbers