California Code, Probate Code - PROB § 4544

(a) Subject to subdivision (b), at least 15 days before the time set for hearing, the petitioner shall serve notice of the time and place of the hearing, together with a copy of the petition, on the following:

(b) In the case of a petition to compel a third person to honor the authority of an attorney-in-fact, notice of the time and place of the hearing, together with a copy of the petition, shall be served on the third person in the manner provided in Chapter 4 (commencing with Section 413.10) of Title 5 of Part 2 of the Code of Civil Procedure.