The Restriction-Extension Operator on Lebesgue spaces with symmetries and applications to PDEs

We prove $L^p$-$L^q$-estimates for the Restriction-Extension operator acting on block-radial functions with the aid of new oscillatory integral estimates and interpolation results in mixed Lorentz spaces. Similar techniques apply to the Limiting Absorption Principles for certain elliptic (pseudo-)differential operators with constant coefficients. In this way we obtain a richer existence theory for Helmholtz-type problems on ${\mathbb{R}}^d$ with block-radial right hand sides.