What do we learn from "Ervas Avicha" in this Pasuk?
Rashi: It forbids his father's wife after his father's death. 1
Ramban, Moshav Zekenim #1 2 : It is the explanation of "Ervas Eshes Avicha" (the wife of his father who is not his mother).
Moshav Zekenim #2: According to R. Yehoshua, it teaches that you are liable for her only for Eshes Av, and not for Eshes Ish. According to Rabanan, it is a second Chiyuv for a man who had Bi'ah with his father.