The extended real number line is obtained from the real number line R by adding two elements: +∞ and -∞ (which are not considered to be real numbers). It is useful in mathematical analysis, especially in integration theory. The extended real number line is denoted by R or [-∞,+∞].
The extended real number line turns into a totally ordered set by defining -∞ ≤ a ≤ +∞ for all a. This order has the nice property that every subset has a supremum and an infimum: it is a complete lattice. The total order induces a topology on R. In this topology, a set U is a neighborhood of +∞ if and only if it contains a set {x : x ≥ a} for some real number a, and analogously for the neighborhoods of -∞. R is a compact Hausdorff space homeomorphic to the unit interval [0,1].
The arithmetical operations of R can be partly extended to R as follows:
- a + ∞ = ∞ + a = ∞ if a ≠ -∞
- a - ∞ = -∞ + a = -∞ if a ≠ +∞
- a × +∞ = +∞ × a = +∞ if a > 0
- a × +∞ = +∞ × a = -∞ if a < 0
- a × -∞ = -∞ × a = -∞ if a > 0
- a × -∞ = -∞ × a = +∞ if a < 0
- a / ±∞ = 0 if -∞ < a < +∞
- ±∞ / a = ±∞ if a > 0
- +∞ / a = -∞ if a < 0
- -∞ / a = +∞ if a < 0
Note that with these definitions, R is not a field and not even a ring. However, it still 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 × (b × c) and (a × b) × c are either equal or both undefined
- a × b and b × a are either equal or both undefined
- a × (b + c) and (a × b) + (a × c) are either equal or both undefined
- if a ≤ b and if both a + c and b + c are defined, then a + c ≤ b + c
- if a ≤ b and c > 0 and both a × c and b × c are defined, then a × c ≤ b × c.
By using the intuition of limits, several functions can be naturally extended to R. For instance, one defines exp(-∞) = 0, exp(+∞) = +∞, ln(0) = -∞, ln(+∞) = ∞ etc.
