Are all convex hulls polytopes too?

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A convex hull of a set $X$ of points is defined as:

the smallest convex set that contains $X$.

Does this definition imply that all convex hulls of a convex set are in fact, polytopes of their respective dimensions?

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Your question seems to me to indicate several confusions.

Here are some facts you should understand. They follow from the definition of "convex hull".

  • A set has just one convex hull, so you needn't ask about "all convex hulls".

  • If a set is convex then it is its convex hull (as @YvesDaoust comments).

  • The convex hull of a finite set will be a polytope. Its dimension may be lower than the dimension of the containing space - for an example think about the convex hull of two points in the plane (or even the convex hull of one point on the line).

  • Any polytope is the convex hull of the (finite) set of its vertices.

  • The convex hull of (the circumference of) a circle in the plane is the circle together with its interior. Clearly that's not a polytope.