Frequency-explicit approximability estimates for time-harmonic Maxwell's equations

We consider time-harmonic Maxwell's equations set in a heterogeneous medium with perfectly conducting boundary conditions. Given a divergence-free right-hand side lying in L-2, we provide a frequency-explicit approximability estimate measuring the difference between the corresponding solution and its best approximation by high-order Nedelec finite elements. Such an approximability estimate is crucial in both the a priori and a posteriori error analysis of finite element discretizations of Maxwell's equations, but the derivation is not trivial. Indeed, it is hard to take advantage of high-order polynomials given that the right-hand side only exhibits L-2 regularity. We proceed in line with previously obtained results for the simpler setting of the scalar Helmholtz equation and propose a regularity splitting of the solution. In turn, this splitting yields sharp approximability estimates generalizing known results for the scalar Helmholtz equation and showing the interest of high-order methods.