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Gruber P. Convex and Discrete Geometry

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7 Valuations 113<br />

where on the right side φ means the given valuation on S. By (4), this function φ is<br />

well defined <strong>and</strong> extends the given valuation. We show that<br />

(6) φ is a valuation on L(S).<br />

For the proof of (6) we will derive two identities. As a preparation for the proof<br />

of the first identity, we prove the following proposition.<br />

(7) Let L be a non-empty finite set. Then �<br />

(−1) |J|+|K | =−(−1) |L| .<br />

J,K �=∅<br />

J∪K =L<br />

For simplicity, we omit, in all sums in the proof of (7), (−1) |J|+|K | . The proof is by<br />

induction on |L|.For|L| =1, Proposition (7) is trivial. Assume now that |L| =l > 1<br />

<strong>and</strong> that (7) holds for l − 1. We may suppose that L ={1, 2,...,l}. Then<br />

�<br />

= �<br />

+ �<br />

+ �<br />

.<br />

J,K �=∅<br />

J∪K =L<br />

1∈J,K<br />

J∪K =L<br />

Next note that<br />

�<br />

= �<br />

1∈J,K<br />

J∪K =L<br />

�<br />

1∈J,�∈K<br />

J∪K =L<br />

�<br />

1�∈J,1∈K<br />

J∪K =L<br />

J={1},1∈K<br />

J∪K =L<br />

= �<br />

J={1},1�∈K<br />

J∪K =L<br />

1∈J,�∈K<br />

J∪K =L<br />

+ �<br />

1∈J,K ={1}<br />

J∪K =L<br />

+ �<br />

{1}� J,1�∈K<br />

J∪K =L<br />

1�∈J,∈K<br />

J∪K =L<br />

+ �<br />

{1}� J,K<br />

J∪K =L<br />

=2(−1) |L|+1 − (−1) |L|−1+2 ,<br />

=(−1) |L| − (−1) |L|−1+1 ,<br />

= =(−1) |L| − (−1) |L|−1+1 ,<br />

by induction. Now we add to get (7). The induction is complete.<br />

The first required identity is as follows:<br />

(8) Let C1 ∪···∪Cm, D = D1 ∪···∪ Dn ∈ S. Then<br />

φ � (C1 ∪···∪Cm) ∩ (D1 ∪···∪Dn) � =− �<br />

I,J<br />

(−1) |I |+|J|−1 φ(CI ∩ DJ ).<br />

The proof is by induction on m. Assume first that m = 1. Then<br />

φ � C1 ∩ (D1 ∪···∪ Dn) � = φ � (C1 ∩ D1) ∪···∪(C1 ∩ Dn) �<br />

= �<br />

(−1) |J|−1 φ(C1 ∩ DJ ) =− �<br />

J<br />

I ={1}<br />

J⊆{1,...,n}<br />

(−1) |I |+|J|−1 φ(CI ∩ DJ )<br />

by the definition of φ on L(S) <strong>and</strong> since I ={1} is the only possibility for I .This<br />

settles (8) for m = 1. Now let m > 1 <strong>and</strong> assume that the identity (8) holds for<br />

m − 1. Then<br />

φ � (C1 ∪···∪Cm) ∩ (D1 ∪···∪ Dn) �<br />

= φ �� C1 ∩ (D1 ∪···∪ Dn) � ∪ � (C2 ∪···∪Cm) ∩ (D1 ∪···∪ Dn) ��<br />

= φ � (C1 ∩ D1) ∪···∪(C1 ∩ Dn) ∪ �<br />

(Ci ∩ D j = Eij, say) �<br />

i∈{2,...,m}<br />

j∈{1,...,n}

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