Elsevier, Journal of Differential Equations, 6(257), p. 1689-1720, 2014
DOI: 10.1016/j.jde.2014.04.021
International Press, Mathematical Research Letters, 6(18), p. 1037-1050
DOI: 10.4310/mrl.2011.v18.n6.a1
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In the euclidean space, Sobolev and Hardy-Littlewood-Sobolev inequalities can be related by duality. In this paper, we investigate how to relate these inequalities using the flow of a fast diffusion equation in dimension $d\ge3$. The main consequence is an improvement of Sobolev's inequality when $d\ge5$, which involves the various terms of the dual Hardy-Littlewood-Sobolev inequality. In dimension $d=2$, Onofri's inequality plays the role of Sobolev's inequality and can also be related to its dual inequality, the logarithmic Hardy-Littlewood-Sobolev inequality, by a super-fast diffusion equation.