I need some light in this exercise:
"If $f:\mathbb {R^n}\rightarrow \mathbb {R^n}$ is continuous differentiable and the image of $f(\mathbb {R^n})$ has $\text{int} =\varnothing$ then the determinant of the Jacobian matrix is zero."
I really don't know where to start... I appreciate any help that helps me to get started.