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Gruber P. Convex and Discrete Geometry

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114 <strong>Convex</strong> Bodies<br />

= �<br />

(−1) |J|−1 φ(C1 ∩ DJ )<br />

J⊆{1,...,n}<br />

+ �<br />

+<br />

J⊆{1,...,n}<br />

�<br />

(−1) |J|<br />

K ⊆{2,...,m}×{1,...,n}<br />

�<br />

K ⊆{2,...,m}×{1,...,n}<br />

(−1) |K |−1 φ(EK )<br />

(by the definition of φ on L(S))<br />

=− �<br />

I ⊆{1}<br />

J⊆{1,...,n}<br />

+ �<br />

J⊆{1,...,n}<br />

+ φ � �<br />

i∈{2,...,m}<br />

j∈{1,...,n}<br />

(−1) |I |+|J|−1 φ(CI ∩ DJ )<br />

(−1) |J| φ � �<br />

(Ci ∩ D j) �<br />

i∈{2,...,m}<br />

j∈{1,...,n}<br />

(−1) |K |−1 φ(C1 ∩ DJ ∩ EK )<br />

(C1 ∩ DJ ∩ Ci ∩ D j) �<br />

(by the definition of φ on L(S))<br />

=− �<br />

(−1) |I |+|J|−1 φ(CI ∩ DJ ) − �<br />

I ⊆{1}<br />

J⊆{1,...,n}<br />

− �<br />

J⊆{1,...,n}<br />

�<br />

|J|<br />

(−1)<br />

I ⊆{2,...,m}<br />

K ⊆{1,...,n}<br />

(by induction, applied twice)<br />

=−<br />

�<br />

−<br />

=−<br />

I ⊆{1} or I ⊆{2,...,m}<br />

J⊆{1,...,n}<br />

�<br />

I ∩{1} �= ∅<br />

I ∩{2,...,m} �= ∅<br />

J ⊆{1,...n}<br />

K ⊆{1,...,n}<br />

�<br />

I ⊆{1} or I ⊆{2,...,m}<br />

J⊆{1,...,n}<br />

+ �<br />

I ∩{1}�=∅<br />

I ∩{2,...,m}�=∅<br />

I ⊆{2,...,m}<br />

J⊆{1,...,n}<br />

(−1) |I |+|J|−1 φ(CI ∩ DJ )<br />

(−1) |I |+|K |−1 φ(C1 ∩ CI ∩ DJ ∩ DK )<br />

(−1) |I |+|J|−1 φ(CI ∩ DJ )<br />

(−1) |I |−1+|J|+|K |−1 φ(CI ∩ DJ ∪ K<br />

(−1) |I |+|J|−1 φ(CI ∩ DJ )<br />

|I |−1<br />

(−1) �<br />

L⊆{1,...,n}<br />

� �<br />

J∪K =L<br />

(by Proposition (7))<br />

=− �<br />

(−1) |I |+|L|−1 φ(CI ∩ DL).<br />

I ⊆{1,...,m}<br />

L⊆{1,...,n}<br />

(−1)<br />

� �� �<br />

=L<br />

)<br />

|J|+|K |<br />

� �� �<br />

=−(−1) |L|<br />

� φ(CI ∩ DL)

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