# Level sets as smooth curves

• yifli
In summary, the theorem states that if U is an open set in ℝ^2, F is a differentiable function on U with a Lipschitz derivative, and X_c is the set of points in U where F(x)=c, then X_c will be a smooth curve if the differential of F at any point a in X_c is onto, meaning it maps to a non-zero value. This is equivalent to saying that the differential is not equal to zero, as seen in simple functions like f(x)=x^2.

#### yifli

I have difficulty understanding the following Theorem

If U is open in $ℝ^2$, $F: U \rightarrow ℝ$ is a differentiable function with Lipschitz derivative, and $X_c=\{x\in U|F(x)=c\}$, then $X_c$ is a smooth curve if $[\operatorname{D}F(\textbf{a})]$ is onto for $\textbf{a}\in X_c$; i.e., if $$\big[ \operatorname{D}F\bigl( \begin{smallmatrix}a \\ b\end{smallmatrix}\bigr)\big]≠0 \mbox{ for all } \textbf{a}=\bigl( \begin{smallmatrix}a \\ b \end{smallmatrix}\bigr)\in X_c$$

I don't understand why the differential of F at a being onto is equivalent to saying the differential is not zero. Can someone explain? Thanks

Think about what happens for simple functions like $f(x)= x^2$ where f'(x)= 0.

HallsofIvy said:
Think about what happens for simple functions like $f(x)= x^2$ where f'(x)= 0.

the differential of $f(x)=x^2$ is $2x$, so $f'(x)=0$ means x=0.
Now what?

## 1. What are level sets?

Level sets refer to the set of points in a multi-dimensional space that have the same value for a given function. They can also be thought of as the curves or surfaces that represent the points where the function has a constant value.

## 2. How are level sets useful in scientific research?

Level sets are useful for visualizing and analyzing complex functions in multi-dimensional spaces. They can help in understanding the behavior and patterns of the function, and can also aid in optimization and modeling in various scientific fields such as physics, engineering, and computer science.

## 3. Can level sets be represented as smooth curves?

Yes, level sets can be represented as smooth curves. This means that the curves have no sharp corners or discontinuities, and the function values change gradually along the curve. Smooth level sets are particularly useful in computer graphics and visualization applications.

## 4. How are level sets different from contour lines?

Level sets and contour lines are similar in that they both represent points with the same value for a given function. However, contour lines are typically used in two-dimensional spaces, while level sets can be used in any number of dimensions. Additionally, contour lines are typically represented as discrete lines, while level sets are continuous curves.

## 5. Are level sets only applicable to scalar functions?

No, level sets can also be applied to vector-valued functions and can be represented as surfaces in three-dimensional spaces. These surfaces are known as level surfaces and can be thought of as the generalization of level sets to higher dimensions.