We give a characterization of Possonian domains in$R$^{$n$}, i.e., those domains for which every bounded harmonic function is the harmonic extension of some function in$L$^{∞}of harmonic measure. We deduce several properties of such domains, including some results of Mountford and Port. In two dimensions we give an additional characterization in terms of the logarithmic capacity of the boundary. We also give a necessary and sufficient condition for the harmonic measures on two disjoint planar domains to be mutually singular.

A sharp result on global small solutions to the Cauchy problem $$u_t = \Delta u + f\left( {u,Du,D^2 u,u_t } \right)\left( {t > 0} \right),u\left( 0 \right) = u_0 $$

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On déduit des estimations gaussiennes supérieures du noyau de la chaleur des estimations du même type pour les premières dérivées spatiales. On obtient ainsi des estimations gaussiennes inférieures du noyau de la chaleur. On donne des applications de ces résultats.

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