Is an oath of divorce valid if it was uttered in a state of anger, and the wife forgot about it and marital relations resumed thereafter? And does it count as a single divorce that necessitates reconciliation or expiation to restore the marriage?

If a man divorces his wife with a revocable divorce, he can take her back before the 'iddah (waiting period) expires, either by word or by action (such as kissing or intercourse). As for the wife forgetting the divorce oath, it is not given consideration by the majority of scholars, and the divorce takes effect. However, the Shafi'is and Ibn Taymiyyah hold that it does not take effect. In this case, the divorce has taken effect according to the majority opinion. But if the husband has touched or had intercourse with his wife during her 'iddah, then that is considered a revocation (of the divorce).