Generalized fractional maximal functions in Lorentz spaces

In this paper we give the complete characterization of the boundedness of the generalized fractional maximal operator $M_{ϕ,Λ^α(b)}f(x) : = \sup_{Q \ni x} \frac{\|f χ_Q\|_{Λ^α(b)}}{ϕ(|Q|)} \qquad (x \in {\mathbb R}^n),$ between the classical Lorentz spaces $Λ^p (v)$ and $Λ^q(w)$ for appropriate functions $ϕ$, where $0 < p,\,q < \infty$, $0 < α\le r < \infty$, $v,w,\,b$ are weight functions on $(0,\infty)$ such that $0 < B(x): = \int_0^x b < \infty$, $x > 0$, $B \in Δ_2$ and $B(t) / t^{α/ r}$ is quasi-increasing.

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