Question: Ulugh Beg and Cahit Arf were both a what?
Sentence (from "Cahit Arf"): He is known for the Arf invariant of a quadratic form in characteristic 2 (applied in knot theory and surgery theory) in topology, the Hasse–Arf theorem in ramification theory, Arf semigroups, and Arf rings.
Relevant evidence: No