Modulus in Banach function spaces

Abstract

Moduli of path families are widely used to mark curves which may be neglected for some applications. We introduce ordinary and approximation modulus with respect to Banach function spaces. While these moduli lead to the same result in reflexive spaces, we show that there are important path families (like paths tangent to a given set) which can be labeled as negligible by the approximation modulus with respect to the Lorentz $L^{p,1}$-space for an appropriate $p$, in particular, to the ordinary $L^1$-space if $p=1$, but not by the ordinary modulus with respect to the same space.