Question: When did Leon Takhtajan's father die?
Sentence (from "Leon Takhtajan"): Takhtajan, son of the Armenian Soviet botanist Armen Takhtajan, received in 1975 his Ph.D. (Russian candidate degree) from the Steklov Institute under Ludvig Faddeev with thesis "Complete Integrability of the Equation formula_1".
Relevant evidence: Yes