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The extended real line

For the purposes of limit theory it's useful to extend the set of real numbers with two other elements, that we write -∞ and +∞, and call, respectively, minus infinity, plus infinity. This extension is useful to treat functions or variables that grow without bounds. This extended set is called the extended real line. The usual order between "ordinary" real numbers is preserved and we define: -∞ < x < +∞, for all reals x. Sometimes we'll also use the symbol ∞, without any sign plus or minus, when it is unimportant to distinguish between the "+" and the "-".

Also the ordinary operations between numbers are extended, with some important exclusions, to this set, by means of the following definitions, where a is a real number, and A is a real number different from zero.

Observe that the following operations are not defined:

Remember that +∞ and -∞ are not numbers: do not write x = ±∞, if x is a number.

It is also of the utmost importance to observe that the extended real line, which we sometimes write img, is not a set of numbers in the ordinary sense. In fact the operations we have introduced do not satisfy many of the usual properties. For instance while the result of a+b changes if you change either a or b, the operation a+(+∞) has always the same result, no matter what a is: 3+(+∞) = 4+(+∞) = (+∞) +(+∞) = (+∞).

Some examples

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first published on march 26 2002 - last updated on february 01 2005