Extended real number line

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

The expressions ∞ − ∞, 0 × ±∞ and ±∞ / ±∞ are usually left undefined. Also, 1 / 0 is not defined as +∞ (because −∞ would be just as good a candidate). These rules are modeled on the laws for infinite limits.

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.

Referenced By