Record number adds up to 4m digits

The biggest prime number - a figure that can only be divided by itself and one - has been discovered by a computer belonging to a 20-year-old Canadian.

It contains 4,053,946 digits and would take three weeks just to write out in longhand. That figure could soon seem like small fry - there is a $100,000 (£730,000) prize on offer for the first person or group to find a 10-milliondigit prime number.

The number, 2 to the power of 13,466,917 - 1, popped up in a co-operative computer project called Gimps or Great Internet Mersenne Prime Search.

It is called a Mersenne prime because it has the particular form of 2 to the power of P - 1, where P is itself a prime number. Only 39 have been found; the first four have P as 3, 7, 31 and 127. The new number is the 39th.

Mersenne primes are valued by mathematicians because they help in the study of encryption and codes, which has become of increasing concern with the surge in use of the internet.

The man whose computer came up with the number (a process that took 45 days), Michael Cameron, said: "A friend informed me that if I was going to leave my computer on all the time I should make use of all that wasted CPU time.

"I put Gimps on my PC because it does not interfere with my work on the computer. Finding the new prime was a wonderful surprise."