Calculus

Limits and Continuity of Functions

Definition of Limit of a Function

Cauchy and Heine Definitions of Limit

Let \(f\left( x \right)\) be a function that is defined on an open interval \(X\) containing \(x = a\). (The value \(f\left( a \right)\) need not be defined.)

The number \(L\) is called the limit of function \(f\left( x \right)\) as \(x \to a\) if and only if, for every \(\varepsilon \gt 0\) there exists \(\delta \gt 0\) such that

\[\left| {f\left( x \right) – L} \right| \lt \varepsilon ,\]

whenever

\[0 \lt \left| {x – a} \right| \lt \delta .\]

This definition is known as \(\varepsilon-\delta-\) or Cauchy definition for limit.

There’s also the Heine definition of the limit of a function, which states that a function \(f\left( x \right)\) has a limit \(L\) at \(x = a\), if for every sequence \(\left\{ {{x_n}} \right\}\), which has a limit at \(a,\) the sequence \(f\left( {{x_n}} \right)\) has a limit \(L.\) The Heine and Cauchy definitions of limit of a function are equivalent.

One-Sided Limits

Let \(\lim\limits_{x \to a – 0} \) denote the limit as \(x\) goes toward \(a\) by taking on values of \(x\) such that \(x \lt a\). The corresponding limit \(\lim\limits_{x \to a – 0} f\left( x \right)\) is called the left-hand limit of \(f\left( x \right)\) at the point \(x = a\).

Note that the \(2\)-sided limit \(\lim\limits_{x \to a} f\left( x \right)\) exists only if both one-sided limits exist and are equal to each other, that is \(\lim\limits_{x \to a – 0}f\left( x \right) \) \(= \lim\limits_{x \to a + 0}f\left( x \right) \). In this case,