Weak logarithmic Sobolev inequalities and entropic convergence

In this paper we introduce and study a weakened form of logarithmic Sobolev inequalities in connection with various others functional inequalities (weak Poincar'{e} inequalities, general Beckner inequalities…). We also discuss the quantitative behaviour of relative entropy along a symmetric diffusion semi-group. In particular, we exhibit an example where Poincar'{e} inequality can not be used for deriving entropic convergence whence weak logarithmic Sobolev inequality ensures the result.