# Find an example of a function f : [ a , b ] &#x2192; R which is contin

Find an example of a function $f:\left[a,b\right]\to R$ which is continuous, but not strictly increasing, such that no inverse function ${f}^{-1}$ satisfy the property of the Inverse Function Theorem.
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Harold Cantrell
Any constant function $f\left(x\right)=c$ will be an example. $f$ is continuously differentiable, but ${f}^{-1}$ can't be a function as you would have ${f}^{-1}\left(c\right)=x$ and ${f}^{-1}\left(c\right)={x}^{\prime }$ where $x\ne {x}^{\prime }$, thereby violating the definition of a function.

Damon Stokes
Consider the function $f\left(x\right)={x}^{2}\mathrm{sin}\left(\frac{1}{x}\right)$ defined on $\left[0,1\right]$. Then you can see that ${f}^{\prime }\left(0\right)=1$ but ${f}^{\prime }$ fails to be continuously differentiable as ${f}^{\prime }$ is not continuous at $0$. And also you can see that there is no neighborhood of $0$ such that ${f}^{-1}$ exists