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Nissim Ben Jacob (Rav Nissim Gaon, 990-1062, Hebrew: ניסים בן יעקב) was a rabbi and Talmudist best known today for his Talmudic commentary "HaMafteach", by which title he is also known.

Rav Nissim studied at the Kairouan yeshiva, initially under his father - Jacob ben Nissim ("Rav Yaakov Gaon") who had studied under Hai Gaon - and then under Chushiel, who succeeded Jacob as head of the Yeshiva. Nissim himself later became head of the yeshiva; in this capacity he is closely associated with Chananel son of Chushiel. His most famous student is probably Isaac Alfasi. Rav Nissim maintained an active correspondence with Hai Gaon and with Shmuel Hanaggid, whose son Joseph married Nissim's only daughter. Some consider Nissim and Chananel to constitute the first generation of Rishonim.

The commentary Sefer Mafteach Manulei Hatalmud (Hebrew: "The book of the key to the Talmud"; often, simply HaMafteach) is essentially a Talmudic cross-reference. In it Rav Nissim identifies the sources for Mishnaic quotes, identifying obscure allusions to other places in Talmudic literature. He quotes from the Tosefta, Mekhilta, Sifre, Sifra, and from the Jerusalem Talmud, the explanations of which he sometimes prefers to those of the Babylonian Talmud. Nissim did not confine himself to quoting references, he also discusses these in connection with the text; this work is thus also a commentary. The work was written on several tractates, and is printed, in many editions, on the page itself.

R. Nissim also wrote other works, some of which have been lost, but which are quoted by later sages:

• "Siddur Tefillah", a siddur (prayerbook)
• A (now lost) commentary on the Torah
• A "Sefer ha-Mitzvot" on the commandments (now lost)
• "Hilkhot Lulav" a polemic against the Karaites (now lost)
• "Megillat Setarim": a collection of notes concerning halakhic decisions, explanations, and midrashim, primarily a note-book for the author's private use, and published by his pupils probably not until after his death.
• A collection of tales, "Sefer Ma'asiyyot ha-Hakhamim": about sixty tales, based upon the Mishnah, Baraita, the two Talmuds, and the midrashic writings; and written at the request of Nissim's father-in-law, Dunash, on the loss of his son.

• Nissim ben Jacob ibn Shahin, tr. William M. Brinner, An Elegant Composition concerning Relief after Adversity: Yale 1977 (Yale Judaica Series vol 20)

• Nissim Ben Jacob Ben Nissim Ibn Shahin, jewishencyclopedia.com
• Nissim Ben Jacob, jewishhistory.org
• Sefer ha-Mafteah, Prof. Eliezer Segal
• R' Nissim ben Yaakov z"l, torah.org

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