# Introduction to Mathematical Physics/Energy in continuous media/Exercises

**Exercice:**
Find the equation evolution for a rope clamped between two walls.

**Exercice:**
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**Exercice:**
Give the expression of the deformation energy of a smectic (see section secristliquides for the description of smectic) whose i layer's state is described by surface ,

**Exercice:**
Consider a linear, homogeneous, isotrope material. Electric susceptibility introduced at section secchampdslamat allows for such
materials to provide from by simple convolution:

eqexsusc

where represents a temporal convolution. To obey to the causality principle distribution has to have a positive support. Indeed, can not depend on the future values of . Knowing that the Fourier transform of function "sign of " is where is a
normalization constant and is the principal value of distribution, give the relations between the real part and imaginary part of the Fourier transform of . These relations are known in optics as *Krammers--Kr\"onig* relations\index{Krammers--Kr\"onig relations}.