The Soviet-born mathematician once taught at colleges in the US
Russian maths genius Grigory Perelman, who declined a prestigious international award four years ago, is under new pressure to accept a prize.
A US institute wants to give him $1m (£700,000) for solving one of the world's most complex mathematical problems, the Poincare Conjecture.
But it is unclear whether Dr Perelman, a virtual recluse, will pick it up.
A children's charity in St Petersburg, where he lives, has urged him to take the money and give it to charity.
Dr Perelman, 43, has cut himself off from the outside world for the past four years, living with his elderly mother in a tiny flat said by neighbours to be infested with cockroaches.
In an open letter on its website, the Warm Home charity called on Dr Perelman to give the cash equivalent of the US Clay Mathematics Institute's $1m Millennium Prize to Russian charities.
It suggested that the mathematician had already made an ethical point by turning down the Fields Medal, the world's highest prize in mathematics, in 2006.
Appeal for privacy
The mathematician is reported to have said "I have all I want" when contacted by a reporter this week about the Clay Millennium Prize.
Grigory Perelman already won maths accolades as a teenager
According to the UK's Daily Mail newspaper, he was speaking through the closed door of his flat.
Dr Perelman was the first person to turn down the Fields Medal, which would have been presented to him at the International Congress of Mathematicians in Madrid.
"I'm not interested in money or fame," he is quoted to have said at the time.
"I don't want to be on display like an animal in a zoo. I'm not a hero of mathematics. I'm not even that successful; that is why I don't want to have everybody looking at me."
One of Russia's most senior politicians, Federation Council Chairman Sergei Mironov, has appealed for Dr Perelman to be left in peace to make up his own mind.
He suggested that it was "not very decent to look into other people's pockets and count other people's money", Russia's Interfax news agency reports.