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Diffusion Equations and Geometric Inequalities
Authors:Borell  Christer
Institution:(1) Department of Mathematics, Chalmers University of Technology and Göteborg University, S-412 96 Göteborg, Sweden
Abstract:Let theta =(theta0, theta1) be a fixed vector in R 2 with strictly positive components and suppose sgr0, sgr1 > 0. Set sgrtheta = theta0 sgr0 + theta1 sgr1 and, if x 0, x 1 isin R n , set x theta = theta0 x 0 + theta1 x 1. Moreover, for any j isin{0, 1, theta}, let c j : R n rarr R be a continuous, bounded function and denote by p sgr j , c j (t, x, y) the fundamental solution of the diffusion equation

$$\frac{{\partial \upsilon }}{{\partial t}} = \frac{{\sigma _j^2 }}{2}\Delta \upsilon - \frac{1}{{\sigma _j^2 }}cjx\upsilon ,t >0,x \in {\mathbf{R}}^n$$
If

$$\frac{1}{{\sigma \theta }}c_\theta \left( {x_\theta } \right) \leqslant \frac{{\theta _0 }}{{\sigma _0 }}\left( {x_0 } \right) + \frac{{\theta _1 }}{{\sigma 1}}c_1 \left( {x_1 } \right),x_0 ,x_1 \in {\mathbf{R}}^n$$
then by applying the Girsanov transformation theorem of Wiener measure it is proved thatsgr n theta p sgr theta, c theta(t, x theta, y theta) ge{sgr n 0 p sgr 0, c 0(t, x 0, y 0)}theta 0 sgr0 / sgrtheta{sgr n 1 p sgr 1, c 1(t, x 1, y 1)}theta 1 sgr1 / sgrtheta for all x 0, x 0, y 0, y 1 isin R n and t > 0. Finally, in the last section, another proof of this inequality is given more in line with earlier investigations in this field.
Keywords:Brownian motion  Hamilton–  Jabobi–  Bellman equation  Girsanov transformation  Brunn–  Minkowski inequality
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