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Gruber P. Convex and Discrete Geometry

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182 <strong>Convex</strong> Bodies<br />

The proof of (3) is thus complete.<br />

Now, using (3) <strong>and</strong> the rearrangement theorem, it follows that<br />

⎧ � �<br />

⎪⎨ 2 f (x) dx<br />

B<br />

T (B) = sup<br />

⎪⎩<br />

� ⎫<br />

2<br />

⎪⎬<br />

� � �2dx : ...<br />

grad f (x) ⎪⎭<br />

B<br />

⎧ � �<br />

⎪⎨ 2 f<br />

D<br />

≤ sup<br />

⎪⎩<br />

r (x) dx � ⎫<br />

2<br />

⎪⎬<br />

� � �<br />

grad f r 2dx<br />

: ... ≤ T (D),<br />

(x) ⎪⎭<br />

D<br />

where D is the circular disc with centre o <strong>and</strong> A(D) = A(B). Its radius is ϱ =<br />

� 1π A(B) � 1/2 . The solution of the boundary value problem,<br />

is given by<br />

Thus<br />

�<br />

T (D) = 2<br />

D<br />

�v + 2 = 0in intD<br />

v| bd D = 0<br />

v is continuous on D<br />

v| int D is of class C 2<br />

v(x) = ϱ2<br />

2<br />

− �x�2<br />

2<br />

for x ∈ D.<br />

� grad v(x) � 2dx = A(D) 2<br />

2π<br />

= A(B)2<br />

2π<br />

. ⊓⊔<br />

Remark. It is well known that, in the estimate for T (B), equality holds precisely<br />

in case when B is a circular disc. For recent results on rods consisting of plastic<br />

material <strong>and</strong> for additional information, see Talenti [987].<br />

First Principal Frequency of a Clamped Membrane<br />

Let K be a smooth closed Jordan curve in E 2 <strong>and</strong> let B be the compact body with K<br />

as boundary. Consider an elastic, homogeneous vibrating membrane on B clamped<br />

along K . Small vertical vibrations of this membrane are described by a function<br />

v(x, t) : B × R → R which satisfies the wave equation,<br />

(4) c �v = vtt in int B × R<br />

v|K × R = 0<br />

v is continuous on B × R<br />

v| int B × R is of class C 2<br />

where c > 0 is a constant depending on the membrane. If v is a non-trivial solution<br />

of (4) of the form v(x, t) = u(x)e iωt , then u is a solution of the following eigenvalue<br />

problem:

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