About Mubārak ben Eli
His name is "Abu l-Fadl Mevorakh [Mubārak] ben Eli" according to:
"Heresy and the Politics of Community: The Jews of the Fatimid Caliphate" (Conjunctions of Religion and Power in the Medieval Past), by Marina Rustow, see reference 50 at bottom of Page 219 of 472 pages, Publisher: Cornell University Press; 1 edition (July 17, 2008) Language: English ISBN-10: 0801445825 ISBN-13: 978-0801445828
Here is a link to Amazon.com source:
The Nagid/Raʾīs al-Yahūd in Egypt Survey of the Research
The office of the nagid and other Jewish leadership institutions in Egypt from the Fāṭimid period on have been the subject of extensive research in the past century, much of it focusing on the origins of the office and title. The problem is compounded by the fact that although the Fāṭimid caliphs conferred the office of raʾīs al-yahūd (head of the Jews), it was the geonim who styled the individual holding this office as nagid. The question of whether this office was created early on in the Fāṭimid period or developed in response to changing historical circumstances at the end of the eleventh century is still an open one.
Among the early scholars, Jacob Mann concluded, primarily on the basis of literary works, that the title raʾīs al-yahūd was granted by the Fāṭimid authorities to a wealthy Jew of high standing at the caliphal court. Mann assumed that the position had been created to shift Jewish allegiance away from the exilarch in Baghdad, the capital of the Fāṭimids’ ʿAbbāsid rivals. A twelfth-century Syrian letter of appointment reveals that a Jew had been given general leadership over all the Jews living in the Fāṭimid domains, whether Rabbanites, Karaites, or Samaritans. The idea of a general headship comports with the broad Islamic approach of ignoring factional differences and appointing a single individual to represent Jews to the regime, and similarly one person to represent Christians. Other sources, including the mid-eleventh-century Megillat Aḥimaʿaṣ from southern Italy and a responsum of David Ibn Abi Zimra (d. 1573), reveal that Jews recognized a raʾīs al-yahūd and referred to him as nagid. Mann concluded that the office of raʾīs al-yahūd and the title of nagid developed in parallel in the early years of the Fāṭimid regime, but became more firmly established when Maimonides and his descendants held the office in the twelfth century.
In the 1950s, several scholars challenged this view. David Ayalon cast doubt on the reliability of the literary sources without offering an alternative hypothesis. Ayalon’s criticism was subsequently accepted by S. D. Goitein and Mark Cohen, who both marshaled dozens of Geniza documents to demonstrate that the terms nagid and raʾīs al-yahūd appeared only from the end of the eleventh century, when the title nagid was conferred upon the two brothers and court physicians Judah and Mevorakh ben Saʿadya. A letter of appointment very similar to that of the Nestorian catholicos in the ʿAbbāsid caliphate was found in the Geniza, issued to the gaon of the Palestinian yeshiva. Enumerating the gaon’s powers and his authority over the Rabbanite Jews of the Fāṭimid Empire, the letter reveals that the gaon alone was the raʾīs al-yahūd. Goitein and Cohen were later joined by Moshe Gil, who argued that appointment as nagid was an internal Jewish matter by which the role of the gaon was recognized, and the Fāṭimid regime simply gave its stamp of approval.
With the expulsion of the Palestinian yeshiva from Jerusalem in the wake of the Seljuq and Crusader conquests, and the concomitant weakening of the authority of the Palestinian gaon, court Jews in Egypt began to ascend to the office of raʾīs al-yahūd, filling a political and spiritual void previously filled by the gaon. The geonim, struggling to maintain some status in the face of the calamities that had befallen them, awarded the title nagid to their distant brethren to secure support for themselves.
Recently discovered evidence seems to support Mann’s original analysis. Both reinterpreting extant evidence and introducing new documents, Shulamit Sela resurrected the hypothesis that the Fāṭimids appointed a raʾīs al-yahūd over Rabbanites, Karaites, and Samaritans from the very beginning of their rule. The incumbent of this office was recognized and styled nagid by the Jews. Sela argued that Karaites held the post in the second and perhaps the third quarter of the eleventh century. This approach viewed the gaon as the prime Rabbanite judicial and legal authority, as was also recorded in the official letter stipulating the gaon’s powers. Thus, Jewish notables in Egypt were subordinate to the halakhic authority of the gaon, who was himself subordinate to the raʾīs al-yahūd, a high-ranking courtier (such as a physician or treasury official) who was appointed by the imam. The post only became firmly established in the twelfth century, when it was held by members of the Maimonides family, and was well documented for the twelfth and thirteenth centuries. Thus, the titles of raʾīs al-yahūd and nagid did not overlap at first, and the gaon did not necessarily grant the title of nagid to everyone who was appointed raʾīs al-yahūd. Sela’s findings have been received with mixed enthusiasm in the scholarly community.