14.02.2013 Views

Gruber P. Convex and Discrete Geometry

Gruber P. Convex and Discrete Geometry

Gruber P. Convex and Discrete Geometry

SHOW MORE
SHOW LESS

Create successful ePaper yourself

Turn your PDF publications into a flip-book with our unique Google optimized e-Paper software.

104 <strong>Convex</strong> Bodies<br />

To remedy the situation that the quermassintegrals depend on the dimension of<br />

the embedding space, McMullen [708] introduced the intrinsic volumes Vi(C),<br />

defined by<br />

Vd−i(C) =<br />

�d� i<br />

κi<br />

Wi(C) for C ∈ C <strong>and</strong> i = 0,...,d.<br />

Proposition 6.7 shows that the intrinsic volumes depend only on the convex body <strong>and</strong><br />

not on the dimension of the embedding space.<br />

From Steiner’s formula <strong>and</strong> the definition of the (Minkowski) surface area S(·)<br />

below we obtain<br />

In addition,<br />

W0(C) = Vd(C) = V (C), W1(C) = 2<br />

d Vd−1(C) = 1<br />

d S(C),<br />

Wd(C) = κd V0(C) = κd.<br />

Wd−1(C) = κd<br />

2<br />

2<br />

w(C), where w(C) =<br />

d κd<br />

�<br />

S d−1<br />

hC(u) dσ(u)<br />

is the mean width of C.Hereσ is the ordinary surface area measure. There is no particularly<br />

simple proof for this equality, so we prove it as a consequence of Hadwiger’s<br />

functional theorem 7.9, see Corollary 7.1.<br />

Minkowski’s Surface Area<br />

Minkowski [738, 739] introduced (see Fig. 6.4) the following notion of (Minkowski)<br />

surface area S(C):<br />

S(C) = lim<br />

ε→+0<br />

Since, by Steiner’s formula,<br />

V (C + εB d ) − V (C)<br />

.<br />

ε<br />

V (C + εB d ) = V (C) + dW1(C)ε + O(ε 2 ) as ε →+0,<br />

this limit exists <strong>and</strong> equals dW1(C). Together with the Brunn–Minkowski<br />

theorem 8.1, this notion of surface area easily leads to the isoperimetric inequality,<br />

C<br />

ε<br />

C + εB 2<br />

Fig. 6.4. (Minkowski) surface area S(C) ∼ � V (C + εB d ) − V (C) � /ε

Hooray! Your file is uploaded and ready to be published.

Saved successfully!

Ooh no, something went wrong!