Definition:Limit of Real Function

Limit from the Left
Let $$f$$ be a real function defined on an open interval $$\left({a \, . \, . \, b}\right)$$.

Suppose that $$\exists L: \forall \epsilon > 0: \exists \delta > 0: b - \delta < x < b \implies \left|{f \left({x}\right) - L}\right| < \epsilon$$

where $$L, \delta, \epsilon \in \R$$.

That is, for every real positive $$\epsilon$$ there exists a real positive $$\delta$$ such that every real number in the domain of $$f$$, less than $$b$$ but within $$\delta$$ of $$b$$, has an image within $$\epsilon$$ of some real number $$L$$.


 * LimitFromLeft.png

Then $$f \left({x}\right)$$ is said to tend to the limit $$L$$ as $$x$$ tends to $$b$$ from the left, and we write:


 * $$f \left({x}\right) \to L$$ as $$x \to b^-$$

or


 * $$\lim_{x \to b^-} f \left({x}\right) = L$$

This is voiced "the limit of $$f \left({x}\right)$$ as $$x$$ tends to $$b$$ from the left".

Sometimes the notation $$f \left({b^-}\right) = \lim_{x \to b^-} f \left({x}\right)$$ is seen.

Limit from the Right
Let $$f$$ be a real function defined on an open interval $$\left({a \, . \, . \, b}\right)$$.

Suppose that $$\exists L: \forall \epsilon > 0: \exists \delta > 0: a < x < a + \delta \implies \left|{f \left({x}\right) - L}\right| < \epsilon$$

where $$L, \delta, \epsilon \in \R$$.

That is, for every real positive $$\epsilon$$ there exists a real positive $$\delta$$ such that every real number in the domain of $$f$$, greater than $$a$$ but within $$\delta$$ of $$a$$, has an image within $$\epsilon$$ of some real number $$L$$.


 * LimitFromRight.png

Then $$f \left({x}\right)$$ is said to tend to the limit $$L$$ as $$x$$ tends to $$a$$ from the right, and we write:


 * $$f \left({x}\right) \to L$$ as $$x \to a^+$$

or


 * $$\lim_{x \to a^+} f \left({x}\right) = L$$

This is voiced "the limit of $$f \left({x}\right)$$ as $$x$$ tends to $$a$$ from the right".

Sometimes the notation $$f \left({a^+}\right) = \lim_{x \to a^+} f \left({x}\right)$$ is seen.

Limit
Let $$f$$ be a real function defined on an open interval $$\left({a \, . \, . \, b}\right)$$ except possibly at some $$c \in \left({a \, . \, . \, b}\right)$$.

Suppose that $$\exists L: \forall \epsilon > 0: \exists \delta > 0: 0 < \left|{x - c}\right| < \delta \implies \left|{f \left({x}\right) - L}\right| < \epsilon$$

where $$L, \delta, \epsilon \in \R$$.

That is, for every real positive $$\epsilon$$ there exists a real positive $$\delta$$ such that every real number in the domain of $$f$$ within $$\delta$$ of $$c$$ has an image within $$\epsilon$$ of some real number $$L$$.


 * LimitOfFunction.png

Then $$f \left({x}\right)$$ is said to tend to the limit $$L$$ as $$x$$ tends to $$c$$, and we write:


 * $$f \left({x}\right) \to L$$ as $$x \to c$$

or


 * $$\lim_{x \to c} f \left({x}\right) = L$$

This is voiced "the limit of $$f \left({x}\right)$$ as $$x$$ tends to $$c$$".

It can directly be seen that this definition is the same as that for a general metric space.