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Gruber P. Convex and Discrete Geometry

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234 <strong>Convex</strong> Bodies<br />

Corollary 13.1. Let ϕ,ψ : N → R + 2<br />

−<br />

be such that 0 0 is a constant depending on C. Now apply the above irregularity<br />

result. ⊓⊔<br />

13.2 Measures on C?<br />

As seen above, the topological concept of Baire categories is an effective tool<br />

to distinguish between small (meagre) <strong>and</strong> large (non-meagre) sets in C <strong>and</strong> Cp.<br />

Considering this, the following problem arises:<br />

Problem 13.1. Define a geometrically useful measure on C or on Cp which is easy<br />

to h<strong>and</strong>le.<br />

The spaces C <strong>and</strong> Cp are locally compact with respect to their common topologies.<br />

Thus there should be many measures available on these spaces. Unfortunately this<br />

is not so, at least so far. A conjecture of the author that Hausdorff measures with<br />

respect to the metric δ H might do, was readily disproved by Schneider [901]. More<br />

general is a negative result of B<strong>and</strong>t <strong>and</strong> Baraki [66]. In view of these results which<br />

indicate that a solution of the above problem might be difficult, it seems to be worth<br />

while to study the following problem:<br />

Problem 13.2. Given an interesting subset D of C or Cp, for example the set of all<br />

proper convex bodies of class C k , or the set of all proper convex bodies with singular<br />

points, find non-decreasing functions h, k :[0, +∞) →[0, +∞) such that for the<br />

corresponding Hausdorff measures µh,µk with respect to a given metric δ on C or<br />

Cp we have<br />

µh(D) = 0, µk(D) >0.

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