Browning, Charles H. Americans of royal descent, 7th ed. Baltimore : Genealogical Pub. Co., 1986, pg. 372 lists him as Robert I.
French nobility. Eldest son of William the Conqueror and Matilda of Flanders, he succeeded his father as Duke of Normandy, his second brother, William II, ruling in England. Conflict with his brothers eventually lost him Normandy and after his capture by Henry I, his younger brother, he was brought to England and imprisoned for twenty-eight years in a succession of castles, primarily at Devizes and Cardiff (findagrave.com)