Let $g:\mathbb{R}\rightarrow \mathbb{R}$ and $h:\mathbb{R}\rightarrow \mathbb{R}$, be two functions such that $h(x) = sgn(g(x))$. Then select
which of the following is not true?( $\mathbb{R}$ denotes the set of all real numbers, sgn stands for
signum function)
Solution
Let $g:\mathbb{R}\to\mathbb{R}$ and $h:\mathbb{R}\to\mathbb{R}$ be such that
$h(x) = \operatorname{sgn}(g(x))$.
Recall:
$\operatorname{sgn}(t) =
\begin{cases}
1, & t>0\\
0, & t=0\\
-1, & t<0
\end{cases}$
Check each statement:
1) "The domain of $h(x)$ is the same as the domain of $g(x)$."
$\Rightarrow$ True, because $\operatorname{sgn}(g(x))$ is defined for every $x$ where $g(x)$ is defined.
2) "The domain of continuity of $h(x)$ equals the domain of continuity of $g(x) - \{x\in\mathbb{R} : g(x)=0\}$."
At points where $g(x)\neq 0$, $h(x)$ is locally constant ($1$ or $-1$), hence continuous there (provided $g$ itself is continuous).
At points where $g(x)=0$, $h(x)$ jumps from $-1$ to $1$, so it is discontinuous.
$\Rightarrow$
This statement is true.
3) "The domain of $h(x)$ is different from the domain of $g(x)$ at the same point."
Since for every $x$ in the domain of $g$, $h(x)=\operatorname{sgn}(g(x))$ is defined, the domains are exactly the same; they never differ.
$\Rightarrow$ This statement is false.
4) " $h(x)$ is discontinuous at $g(x)=0$."
At any $x_0$ where $g(x_0)=0$, the left and right limits of $h(x)$ are $-1$ and $1$, not equal to $h(x_0)=0$.
$\Rightarrow$ $h$ is discontinuous there, so this statement is true.
Therefore, the statement which is **not true** is:
$\boxed{\text{Option 3}}$