Rav Yitzchak Zilber רבי יצחק זילבר

His father, a Kazan rabbi, left him two charges: learn Torah and never live off the community. He trained as a mathematician—Galois theory, algebra—and for decades solved an impossible problem: Soviet employment without Shabbat. Schedules, shifts, certificates—a whole science, and he built it.

In the labor camp he kept count of days so as not to lose Shabbat, and cooked kasha in his own bowl; free, he taught children Torah in secret—men were jailed for that in the fifties. He got out in 1972.

In Jerusalem he became the father of a generation of Russian baalei teshuva: classes in Russian every day, in any weather, to the last weeks of his life. Toldot Yeshurun is carried on by his son Rav Bentzion. And To Remain a Jew is read by people who never opened another Jewish book.