Carathéodory's theorem (convex hull)

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An illustration of Carathéodory's theorem for a square in R2
See also Carathéodory's theorem (disambiguation) for other meanings

In convex geometry Carathéodory's theorem states that if a point x of Rd lies in the convex hull of a set P, there is a subset Template:Italics correction′ of P consisting of d + 1 or fewer points such that x lies in the convex hull of Template:Italics correction′. Equivalently, x lies in an r-simplex with vertices in P, where . The result is named for Constantin Carathéodory, who proved the theorem in 1911 for the case when P is compact. In 1914 Ernst Steinitz expanded Carathéodory's theorem for any sets P in Rd.

For example, consider a set P = {(0,0), (0,1), (1,0), (1,1)} which is a subset of R2. The convex hull of this set is a square. Consider now a point x = (1/4, 1/4), which is in the convex hull of P. We can then construct a set {(0,0),(0,1),(1,0)} = Template:Italics correction′, the convex hull of which is a triangle and encloses x, and thus the theorem works for this instance, since |Template:Italics correction′| = 3. It may help to visualise Carathéodory's theorem in 2 dimensions, as saying that we can construct a triangle consisting of points from P that encloses any point in P.


Let x be a point in the convex hull of P. Then, x is a convex combination of a finite number of points in P :

where every xj is in P, every λj is non-negative, and .

Suppose k > d + 1 (otherwise, there is nothing to prove). Then, the points x2 − x1, ..., xk − x1 are linearly dependent,

so there are real scalars μ2, ..., μk, not all zero, such that

If μ1 is defined as


and not all of the μj are equal to zero. Therefore, at least one μj > 0. Then,

for any real α. In particular, the equality will hold if α is defined as

Note that α>0, and for every j between 1 and k,

In particular, λi − αμi = 0 by definition of α. Therefore,

where every is nonnegative, their sum is one , and furthermore, . In other words, x is represented as a convex combination of at most k-1 points of P. This process can be repeated until x is represented as a convex combination of at most d + 1 points in P.

An alternative proof uses Helly's theorem.

See also


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