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Gruber P. Convex and Discrete Geometry

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194 <strong>Convex</strong> Bodies<br />

Let σm be the discrete Borel measure on Sd−1 which is defined as follows:<br />

σm(B) = �<br />

σ(Si)ϱi for each Borel set B ⊆ S d−1 .<br />

We show that<br />

ui ∈B<br />

(17) σm converges weakly to σ as m →∞.<br />

Let g : Sd−1 → R be continuous. Then<br />

�<br />

�<br />

g(u) dσm(u) − g(u) dσ(u)<br />

S d−1<br />

By (16),<br />

i=1<br />

S d−1<br />

n�<br />

�<br />

�<br />

= σ(Si)ϱi g(ui) − g(u) dσ(u) � n�<br />

�<br />

�<br />

= ϱi g(ui) − g(u) � dσ(u).<br />

Si<br />

i=1<br />

Si<br />

1<br />

|ϱi g(ui) − g(u)| ≤|g(ui) − g(u)|+ max<br />

u∈Sd−1{|g(u)|} 2m<br />

If u ∈ Si, then �ui − u� ≤diam Si ≤ 1 m . Since g is uniformly continuous on Sd−1 ,<br />

we see that<br />

�<br />

�<br />

g(u) dσm(u) − g(u) dσ(u) → 0asm →∞,<br />

S d−1<br />

concluding the proof of (17).<br />

By (i) <strong>and</strong> (15),<br />

o =<br />

=<br />

�<br />

S d−1<br />

udσ(u) =<br />

S d−1<br />

n�<br />

�<br />

udσ(u) =<br />

i=1<br />

Si<br />

n�<br />

ϱiσ(Si)ui<br />

i=1<br />

n�<br />

αiui, where αi = ϱiσ(Si) >0.<br />

i=1<br />

It follows from (i) that there is 0 0.<br />

i=1<br />

i=1<br />

2 .

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