Radii of regular polytopes

This paper deals with the three types of regular polytopes which exist in all dimensions -- regular simplices, cubes and regular cross-polytopes -- and their outer and inner radii. While the inner radii of regular simplices are well studied, only a few cases are solved for the outer radii. We give a lower bound on these radii, and show that this bound is tight in almost 3 out of 4 dimensions. In a further section we complete the results about inner and outer radii of general boxes and cross-polytopes. Finally, because cubes and regular cross-polytopes are radii-minimal projections of simplices, we show that it is possible to deduce the results about their radii from the results about the outer radii of simplices.