On boundedness of operators of weak type $(\varphi_0, \psi_0, \varphi_1, \psi_1)$ in Lorentz spaces in limit cases

We prove theorems on boundedness of operators of weak type $(\varphi_0, \psi_0, \varphi_1, \psi_1)$ from Lorentz space $\Lambda_{\varphi,a}(\mathbb{R}^n)$ to $\Lambda_{\varphi,b}(\mathbb{R}^n)$ in “limit” cases, when one of functions $\varphi(t) / \varphi_0(t)$, $\varphi(t) / \varphi_1(t)$ slowly changes at zero and at infinity.