What is the ruling on a husband having intercourse with his wife after divorcing her with one revocable pronouncement, without her knowledge, and before her waiting period (iddah) has ended? And is it obligatory to inform her of the divorce?

Intercourse that occurs less than a week after a divorce is permissible, and it effects a رجعة (return to the marriage) according to the Hanafis and Hanbalis, even if the رجعة was not intended. If the رجعة was intended, then the رجعة is valid according to the majority of scholars. The wife must be informed of her divorce once, so that she can protect herself in the future if she is divorced twice, and because in this case, she becomes a stranger (non-mahram).