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Estimation optimale du gradient du semi-groupe de la chaleur sur le ...

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510 J. Van Schaftingen / Journal of Functional Analysis 236 (2006) 490–516<br />

5.4. Integrability of functions in Dk(Rn )<br />

If u ∈ BMO(Rn ), uBMO(Rn ) 1 and <br />

B(0,1) udx = 0, the John–Nirenberg theorem states<br />

that<br />

<br />

exp μ|u| dx c, (5.2)<br />

B(0,1)<br />

where μ>0 and c>0 can be chosen in<strong>de</strong>pen<strong>de</strong>ntly of u. Since the spaces Dk(Rn ) are embed<strong>de</strong>d<br />

in BMO(Rn ), this might be improved on Dk(Rn ).<br />

On the other hand, if sp = n,0

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