On the stability of the Lions-Peetre method of real interpolation with functional parameter

Let $\vec{X}=(X_0, X_1)$ be a compatible couple of Banach spaces, $1\le p \le \infty$ and let $\varphi$ be positive quasi-concave function. Denote by $\overline{X}_{\varphi,p}=(X_0,X_1)_{\varphi,p}$ the real interpolation spaces defined by S. Janson (1981). We give necessary and sufficient conditions on $\varphi_{0}$, $\varphi_{1}$ and $\varphi$ for the validity of \begin{equation*} \left(\overline{X}_{\varphi_{0},1},\overline{X}_{\varphi_{1},1} \right) _{\varphi,p}= \left(\overline{X}_{\varphi_{0},\infty},\overline{X}_{\varphi_{1},\infty}\right)_{\varphi,p} \end{equation*} for all $1\le p\le \infty$, and all Banach couples $\overline{X}.$