Mathematician and Nobel laureate Dr John Nash tells Shane Hegarty about his extraordinary life - and what he thought of it on film

Dr John Nash reckons he might work on for a while yet. "It's better to be working than not working. I'm not sure how much I can do at this age, but there are many cases of farmers who reach 100 and are still working."

Could he see himself working at 100 years old? "Oh sure. But you have to have luck to live that long, even nowadays when it's becoming less rare."

His wife Alicia is listening in from the corner of the room. Can she see him working until he's 100?

"Absolutely."

He's 76 now, and even by the standards of a life that has been anything but ordinary, the past decade has been particularly adventurous. He is one of the most famous mathematicians on the planet, although most people don't know the details. He won the Nobel Prize in 1994 for a mathematical theory that has become a cornerstone of modern economics, but it was Russell Crowe who brought him to the masses. In the movie A Beautiful Mind, Hollywood took Nash's remarkable story of mathematics and schizophrenia and fashioned an unlikely hero from it. He is sanguine about the liberties they took in the process.

"I thought at first the music was too loud," says Nash. "But after I got into it I realised that this movie had the ingredients for success because there's a measure of suspense and an entertaining quality. It was hard to accept the personal description but I could see that while it might not be like a documentary, it could be successful as a movie."

He thinks that Russell Crowe should have won an Oscar for his portrayal of him. It was the wrong accent, and they met only once during filming, but "his performance was terrific". Alicia adds: "We just love Russell Crowe. It was a great movie, but it was fictionalised."

She and her husband then debate whether Jennifer Connelly should have won a leading actress Oscar rather than for supporting actress. "She was the lead," insists Alicia, whom Connelly played in the movie.

They are in Dublin because Nash this week gave a lecture as part of the Royal Irish Academy and The Irish Times series, sponsored by DEPFA Bank, in which he talked about both his work and that of Irish mathematician JL Synge, who taught the young Nash. These are arcane topics, but not for Nash.

"Mathematicians like to speak of it as an art, and having beauty and turning this beauty into a good way of proving and seeing something. And you do really get to the truth - there are unknown things that can become known - and it really is the truth. It is not political propaganda."

The maths in the movie was a little vague, especially on Nash Equilibrium, the concept that made his professional reputation. "In the movie there isn't an explanation. There's something about Adam Smith," he laughs at the notion that it would be linked with the economist and philosopher. "There really isn't an explanation."

It certainly wasn't conceived in a eureka moment while trying to score with some girls, as the movie makes out.

These were not the only questions marks hanging over the account. Journalists wondered why it glossed over the less glamorous aspects of his life: an estranged son from another relationship; alleged bisexuality; a perceived anti-Semitism during schizophrenic episodes.Doctors questioned its depiction of schizophrenia itself.

Today some of the personal aspects are off-limits, but he is honest about his paranoid schizophrenia, first diagnosed in 1950 and which followed him for three decades.

He is generous to the film-makers, who created a visible world of delusions that never actually existed.

"It's inaccurate, but it's sort of interpretative," says Nash. "The screenwriter is really responsible for that, and he had a mother who was a psychiatrist and this is very relevant because he was sort of into the area."

How did it deviate from his experiences? "Typically people with schizophrenia do not see anything. But the movie is a visual medium, so the delusions are seen by the character. But the person doesn't necessarily see anything, but might hear voices. When you think about it, it's hard to do that in a movie."

Nash's illness compelled him to do many things. He believed that aliens were trying to contact him through the New York Times newspaper, he travelled around Europe trying to achieve refugee status and renounce his US citizenship. When he later returned to Princeton, he became known as "the Phantom", a figure seen scribbling away on blackboards late into the night. It led to him being sidelined by the academic community and to divorce from Alicia in 1963.

Yet, he gradually learned to ignore the voices. "I began to tire of certain types of irrational thinking. I was doing things at the time, studying or doing some calculations. So it may be that the delusional thinking began to come unsatisfying. I think people become mentally ill when they're somehow not too happy - not just after you've won the lottery you go crazy. It's when you don't win the lottery."

When he looks back, he sees clues in his early behaviour. "Some decisions that might not have been the most rational. Times I didn't follow the norm, thought differently. But I can see there's a connection between not following normal thinking and doing creative thinking. I wouldn't have had good scientific ideas if I had thought more normally. One could be very successful in life and be very normal, but if you're Van Gogh or artists like that you may be a little off."

His honesty when talking about it is impressive, although he limits his work in the area as he fears it could be exploited. "There are times when I've had letters and requests from various people about someone who has some mental problem in the family and they want to be helped, and I can't really do anything in that specific case. And it's not right around the corner, it's very far away."

Nash has not had an episode for many years, but his son has been ill. "He has a mental problem. I'm not sure that the word schizophrenia and bi-polar disorder . . . there's truly a spectrum. There's two ends and a roadway between them. And some of the same medicines may be similarly effective in either categories."

The stigma against mental illness, he believes, will only be removed when the disease is. "I think that when you try to de-stigmatise you may have some other objectives. The doctors and those who treat people with mental illness, they want to stay in business," he says. "Many people are kept on a maintenance level of medicines for the remainder of their lives. But these medicines can have very bad side effects. It might be very good for a while, but you can only take it for a short time."

Clarity of mind has brought rewards for him late in life. A Nobel laureate, he is now feted by economists, scientists and mathematicians, and enjoys that recognition. His Dublin talk was booked out for weeks in advance. His work is currently concerned with developing his early theories, and in his areas of interest - cosmology, gravitation, logic, game theory - the well has not run dry. "Some of it can be considered fantastic and implausible. But things are possible."

It also brought reconciliation between him and Alicia. They re-married in 2001, an outcome perhaps more romantic than the happy ending of A Beautiful Mind. "You're reminding me of Charles and Camilla now," he grins. "We had been together for a long time, there was a separation and a divorce. Some years passed, and I was in some different locations. Then after 1970 we were living in the same place for a long time. And I got a more regular position and I had an income. It became a different situation. Now she's retired and I have an income."

"I have a pension," adds Alicia quickly.

The interview ends.

"Make sure you say something nice about Russell Crowe," says Alicia.

On the notepad in front of Dr Nash is what appears to be a doodled equation. Does it mean anything? He looks at it. "No. It's only XYZ. I was just testing the pencil."

Nash in a nutshell

Nash Equilibrium originted in 1950. It applies to non-co-operative situations and occurs when no player has an incentive to deviate from the strategy chosen, since no player can choose a better strategy, given the choices of the other players. It is important in economics and behavioural strategies.