Robert Curthose (c. 1050 - 3 February 1134), sometimes numbered Robert II or Robert III, was the Duke of Normandy from 1087 until 1106 and an unsuccessful claimant to the throne of the Kingdom of England In 1087, the Conqueror died of wounds suffered from a riding accident during a siege of Mantes. At his death he reportedly wanted to disinherit his eldest son but was persuaded to divide the Norman dominions between his two eldest sons. To Robert he granted the Duchy of Normandy and to William Rufus he granted the Kingdom of England. The youngest son Henry was given money to buy land. Of the two elder sons Robert was considered to be much the weaker and was generally preferred by the nobles who held lands on both sides of the English Channel since they could more easily circumvent his authority. At the time of their father's death the two brothers made an agreement to be each other's heir. However this peace lasted less than a year when barons joined with Robert to displace Rufus in the Rebellion of 1088. It was not a success, in part because Robert never showed up to support the English rebels.
David, Charles Wendell (1920). Robert Curthose, Duke of Normandy (Reprint of original ed.). Cambridge, MA: Harvard University Press