Leonid Plyushch, a Ukrainian mathematician who became a leading political dissident in the Soviet era, prompting the Kremlin to commit him to a mental asylum for a nightmarish three years of drugs and deprivation in the early 1970s, died on Thursday near Paris.

Mr. Plyushch’s death was announced by Arina Ginzburg, a friend who had also been a Soviet dissident, according to Agence France-Presse. No cause of death was given. Various reports gave Mr. Plyushch’s age as either 76 or 77.

Mr. Plyushch (pronounced, roughly, Plootch) had lived in Paris in recent years, traveling now and then to Kiev, the Ukrainian capital. It was there, decades ago when Ukraine was a Soviet republic, that he emerged as a double annoyance to the Kremlin, not only resisting thought control in general but balking at Moscow’s efforts to smother Ukrainians’ yearning for sovereignty and their sense of cultural separateness from Russia.

It was not unheard-of in those days for Ukrainian artists and intellectuals to meet with “mysterious accidents,” said Nadia Diuk, an expert on Russia and Ukraine and a vice president of the National Endowment for Democracy in Washington.