We present new formulae providing equivalent quasi-norms on Lorentz-Karamata spaces. Our results are based on properties of certain averaging operators on the cone of non-negative and non-increasing functions in convenient weighted Lebesgue spaces. We also illustrate connections between our results and mapping properties of such classical operators as the fractional maximal operator and the Riesz potential (and their variants) on the Lorentz-Karamata spaces.
We characterize compact embeddings of Besov spaces $B^{0,b}_{p,r}(\mathbb {R}^n)$ involving the zero classical smoothness and a slowly varying smoothness $b$ into Lorentz-Karamata spaces $L_{p, q; \bar {b}}(\Omega )$, where $\Omega $ is a bounded domain in $\mathbb {R}^n$ and $\bar {b}$ is another slowly varying function.