Definition:

$\quad \quad $ If the graph of a function has no break or jump, then it is said to be continuous function. A function which is not continuous is called a discontinuous function.

Continuity of a Function at a Point :

$\quad \quad $ A Function $f(x)$ is said to be continuous at some point $x=a$ of its domain if

$ \lim _{x \rightarrow a} f(x)=f(a) $

$\lim _{x \rightarrow a^{-}} f(x)=\lim _{x \rightarrow a^{+}} f(x)=f(a)$

Continuity from Left and Right :

$\quad \quad $ Function $\mathrm{f}(\mathrm{x})$ is said to be

• $\quad $ Left Continuous at $x=a$ if $\lim _{x \rightarrow a^{-}} f(x)=f(a) \quad$

• $\quad $ Right Continuous at $x=a$ if $\lim _{x \rightarrow a^{+}} f(x)=f(a)$

$\quad \quad $ Thus a function $\mathrm{f}(\mathrm{x})$ is continuous at a point $\mathrm{x}=\mathrm{a}$ if it is left continuous as well as right continuous at $\mathrm{x}=\mathrm{a}$.

Continuity in an Interval :

• A function $f(x)$ is continuous in an open interval (a, b) if it is continuous at every point of the interval.

• A function $\mathrm{f}(\mathrm{x})$ is continuous in a closed interval [a, b] if it is continuous in (a, b)

  • right continuous at $\mathrm{x}=\mathrm{a}$

  • left continuous at $\mathrm{x}=\mathrm{b}$

Continuous Functions :

$\quad \quad$ A function is said to be continuous function if it is continuous at every point in its domain. Following are examples of some continuous functions:

• $ f(x)=x$ (Identify function)

• $ f(x)=c$ (Constant function)

• $ f(x)=a_{0} x^{n}+a_{1} x^{n-1}+\ldots . .+a^{n}$ (Polynomial function)

• $ f(x)=\sin x, \cos x$ (Trigonometric function)

• $ f(x)=a^{x}, e^{x}, e^{-x}$ (Exponential function)

• $ f(x)=\log x$ (Logarithmic function)

• $ f(x)=\sinh x, \cosh x, \tanh x$ (Hyperbolic function)

• $ f(x)=|x|, x+|x|, x-|x|, x|x|$ (Absolute value functions)

Discontinuous Functions :

$\quad \quad $ A function is said to be a discontinuous function if it is discontinuous at at least one point in its domain. Following are examples of some discontinuous functions:

Properties of Continuous Functions :

• The sum, difference, product, quotient (if $\mathrm{Dr} \neq 0$ ) and composite of two continuous functions are always continuous functions.

• if $\mathrm{f}(\mathrm{x})$ and $\mathrm{g}(\mathrm{x})$ are continuous functions then following are also continuous functions:

  • $f(x)+g(x)$
  • $ f(x)-g(x)$
  • $f(x) \cdot g(x)$
  • $ \lambda f(x)$, where $\lambda$ is a constant
  • $\frac{\mathrm{f}(\mathrm{x})}{\mathrm{g}(\mathrm{x})}$, if $\mathrm{g}(\mathrm{x}) \neq 0$
  • $\mathrm{f}[\mathrm{g}(\mathrm{x})]$

Important Point :

• The discontinuity of a function $\mathrm{f}(\mathrm{x})$ at $\mathrm{x}=$ a can arise in two ways

  • If $\lim _{x \rightarrow a^{-}} f(x)$ exist but $\neq f(a)$ or $\lim _{x \rightarrow a^{+}} f(x)$ exist but $\neq f(a)$, then the function $f(x)$ is said to have a removable discontinuty.

  • The function $\mathrm{f}(\mathrm{x})$ is said to have an non-removable discontinuity when $\lim _{\mathrm{x} \rightarrow \mathrm{a}} \mathrm{f}(\mathrm{x})$ does not exist.

$ \text { i.e. } \lim _{x \rightarrow a^{-}} f(x) \neq \lim _{x \rightarrow a^{+}} f(x) $

Differentiability at a point

$\quad \quad$ Let $f(x)$ be a real valued function defined on an open interval $(a, b)$ and let $c \in(a, b)$. Then $f(x)$ is said to be differentiable or derivable at $x=c$, iff $\lim _{x \rightarrow c} \frac{f(x)-f(c)}{x-c} \quad \text{exists finitely} $

$\quad \quad$ This limit is called the derivative or differential coefficient of the function $\mathrm{f}(\mathrm{x})$ at $\mathrm{x}=\mathrm{c}$, and is denoted by $\mathrm{f}^{\prime}(\mathrm{c})$

• Left-Hand Derivative (LHD)

$\quad \quad$ If $f(x)$ is defined in some neighborhood of $c$ and the limit $ \lim _{h \rightarrow 0^-} \frac{f(c-h)-f(c)}{h} $ $\quad \quad$ exists, then it is called the left-hand derivative of $f(x)$ at $x = c$ and is denoted by $f’(c^-)$ or $Lf f’(c)$.

• Right-Hand Derivative (RHD)

$\quad \quad$ If $f(x)$ is defined in some neighborhood of $c$ and the limit $ \lim _{h \rightarrow 0^+} \frac{f(c+h)-f(c)}{h} $ $\quad \quad$ exists, then it is called the right-hand derivative of $f(x)$ at $x = c$ and is denoted by $f’(c^+)$ or $Rf f’(c)$.

$\quad \quad$ If $Lf f’(c) \neq Rf f’(c)$, we say that $f(x)$ is not differentiable at $x=c$.

Differentiability in a Set :

$\quad \quad$ A function $f(x)$ defined on an open interval $(a, b)$ is said to be differentiable or derivable in the open interval $(a, b)$ if it is differentiable at each point of $(a, b)$.