What is the standard of conduct of counsel on appeal?

We appointed counsel to represent defendant on appeal. Counsel filed a brief summarizing the proceedings and facts of the case and advised the court he found no arguable issues to assert on defendant's behalf. (Anders v. California (1967) 386 U.S. 738; People v. Wende (1979) 25 Cal.3d 436.) Counsel and this court notified defendant he could file a supplemental brief on his own behalf. However, we received no supplemental brief from him and the time to file one has passed.