Alexis Lemaire slouched back in his chair, crossed his legs and stared at the screen. If he was aware of the chattering noise echoing around the museum, he showed no sign of it.

Staring at a 200-digit number he had never seen before, his thick eyebrows twitched gently, his breathing slowed and he extended the middle finger of his right hand to tap the answer into the computer.

Lemaire's last keystroke - which came exactly 70.218 seconds after he was first confronted with the random figure - revealed the number's 13th root: 2407899893032210.

Those 16 digits, and the remarkably brief time it had taken him to pluck them from his cerebral hardware, yesterday sealed the 27-year-old Frenchman's reputation as the world's fastest human calculator. "This is better than I had expected," Lemaire said.

It was; the tension of the test, at the Science Museum in London, appeared to have spurred him on to beat his own world record by two seconds.

Remaining reassuringly philosophical, the world's top "mathlete" went on to explain his technique for unscrambling the 13th root in roughly the time it takes to scramble eggs. "I have a multiplication table in my head. It's like learning normal multiplication tables but instead of 10 times 10, it's hundreds and thousands of different multiplications."

Lemaire - who works for an artificial intelligence company while studying for his doctorate and still giving the odd class - is one of a handful of individuals with the ability to "see" numbers, memorise countless calculations and process information. This allows them to solve complex mathematical problems in their head - and faster than most could do it using a calculator.

"It's a combination of many skills and abilities; memory, calculations and mathematical skills," he said. "I realised I was good at maths at nine after I was able to find for myself the square root of any eight-digit number I saw on the screen of a calculator."

Lemaire tours the world, competing against himself and trying to outdo his previous personal best. It is hard to know whether his quest is fuelled by adrenaline, ego or obsession. But as far as he is concerned, it is a simple buzz that propels him from august institution to august institution where he tests himself with a laptop before a crowd.

"I do this because it's very exciting when I get the answer right. It helps me improve and it gives me a feeling that's very pleasant and satisfying."

That's not to say that he is totally unnicked by arrogance: "Of course," he added, "I get something psychological from it because no one else can do it." His "secret method" is evidently not something he intends to share.

Preparations for the calculation require four or five hours' training a day. Careful not to overdo it, Lemaire jogs to relieve the stress - and winds down by memorising numbers. His final aim is daunting even by his own unique standards. And beside it, his existing achievements lose some of their lustre.

"My main ambition is to control my brain so that it can calculate everything in life, like language," he explained calmly. "Then I want to upload my mind on to a computer so that it can have the same skills as me."

Plans like that leave little time for a personal life. But despite being a student at the University of Reims, he is not one for toasting his triumphs with champagne and carousing.

"That," he said, looking distractedly at the numbers on the computer, "is not something I do."