We show that there exists, for each closed bounded convex set C in the Euclidean plane with nonempty interior, a quadrangle Q having the following two properties. Its sides support C at the vertices of a rectangle r and at least three of the vertices of Q lie on the boundary of a rectangle R that is a dilation of r with ratio 2. We will prove that this implies that quadrangle Q is contained in rectangle R and that, consequently, the inner approximation r of C has an area of at least half the area of the outer approximation Q of C. The proof makes use of alignment or Schüttelung, an operation on convex sets.
For a finer inner and outer approximation, Brinkhuis REF showed that there exists a quadrangle Q that its sides support C at the vertices of a rectangle r and at least three of its vertices lie on the boundary of a rectangle R that is a dilation of r with ratio 2.
Inner and outer approximation of convex sets using alignment
Optimization Letters
Mathematics Computer Science