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Gruber P. Convex and Discrete Geometry

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138 <strong>Convex</strong> Bodies<br />

In the following, summation on i <strong>and</strong> j is from 0 to d.LetB, C ∈ C with B ∪ C ∈ C.<br />

Then<br />

φ(B ∪ C, D) = �<br />

ci(B ∪ C) Wi(D),<br />

i<br />

φ(B ∩ C, D) = �<br />

ci(B ∩ C) Wi(D),<br />

i<br />

φ(B, D) = �<br />

ci(B) Wi(D),<br />

i<br />

φ(C, D) = �<br />

ci(C) Wi(D) for D ∈ C.<br />

i<br />

This, together with (5), shows that<br />

0 = � �<br />

ci(B ∪ C) + ci(B ∩ C) − ci(B) − ci(C) � Wi(D) for D ∈ C.<br />

i<br />

Taking D = B 0 , B 1 ,...,B d , the vectors � W0(D),...,Wd(D) � form a basis of<br />

E d+1 . Hence<br />

ci(B ∪ C) + ci(B ∩ C) − ci(B) − ci(C) = 0,<br />

concluding the proof of (10). Next:<br />

(11) ci(·) is continuous for i = 0,...,d.<br />

Choose C, C1, C2, ···∈C such that Cn → C as n →∞. Then<br />

�<br />

ci(Cn) Wi(D) = φ(Cn, D) → φ(C, D) = �<br />

ci(C) Wi(D) for any D ∈ C<br />

i<br />

by (9) <strong>and</strong> (8). Hence<br />

� �<br />

ci(Cn) − ci(C) � Wi(D) → 0 for any D ∈ C.<br />

i<br />

Taking D = B 0 , B 1 ,...,B d , the vectors � W0(D),...,Wd(D) � form a basis of<br />

E d+1 . Hence<br />

ci(Cn) − ci(C) → 0.<br />

The proof of (11) is complete. The last required property of the ci says that<br />

(12) ci(·) is invariant with respect to proper rigid motions for i = 0,...,d.<br />

Let C ∈ C, m ∈ M. Then<br />

�<br />

ci(mC) Wi(D) = φ(mC, D) = φ(C, D) = �<br />

ci(C) Wi(D)<br />

i<br />

i<br />

i

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