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Sobolev-type inequalities on Cartan-Hadamard manifolds and applications to some nonlinear diffusion equations

We investigate the validity, as well as the failure, of Sobolev-type inequalities on Cartan-Hadamard manifolds under suitable bounds on the sectional and the Ricci curvatures. We prove that if the sectional curvatures are bounded from above by a negative power of the distance from a fixed pole (times a negative constant), then all the $ L^p $ inequalities that interpolate between Poincaré and Sobolev hold for radial functions provided the power lies in the interval $ (-2,0) $. The Poincaré inequality was established by H.P. McKean under a constant negative bound from above on the sectional curvatures. If the power is equal to the critical value $ -2 $ we show that $ p $ must necessarily be bounded away from $ 2 $. Upon assuming that the Ricci curvature vanishes at infinity, the nonradial version of such inequalities turns out to fail, except in the Sobolev case. Finally, we discuss applications of the here-established Sobolev-type inequalities to optimal smoothing effects for radial porous medium equations.