“…I. Perov-A. I. Povolotskii-P. P. Zabreiko [27,Section 15], B. L. Shekhter [34,Section 15] for a more classical approach in the ODE's case. For other results, in the PDE's setting, we also refer to V. Moroz [29], P. Omari-F. Zanolin [31], E. W. C. Van Groesen [35], M. Willem [36], mainly for the case a-1.…”