Ibrahim ibn al-Mahdi
Ibrahim ibn al-Mahdi (; 779–839) was an Abbasid prince, singer, composer and poet. He was the son of the third Abbasid caliph, al-Mahdi, and the half-brother of the poet and musician Ulayya. He was not a full brother of al-Mahdi's sons al-Hadi and Harun al-Rashid, since his mother was not al-Khayzuran. During the Fourth Fitna, Ibrahim was proclaimed caliph on 20 July 817 by the people of Baghdad, who gave him the regnal name of al-Mubarak () and declared his reigning nephew al-Ma'mun deposed. Ibrahim received the allegiance  of the Hashemites. He had to resign in 819, and spent the rest of his life as a poet and a musician. He is remembered as "one of the most gifted musicians of his day, with a phenomenal vocal range", and a promoter of the then innovative 'Persian style' of song, 'which was characterized "inter alia" by redundant improvisation'.