Computer hits on prime

Press, 19 September 1985, Page 8

Computer hits on prime

NZPA-AP Los Angeles Scientists preparing a socalled super-computer for oil exploration had accidentally stumbled on to the largest prime number discovered, the “Los Angeles Times” reported yesterday. It was the number 2 raised to the 216,0915 t power minus 1, contained 65,050 digits, and would fill two pages if printed in a newspaper, the “Times” said. Prime numbers are numbers that cannot be divided evenly except by themselves and the number one. For example, 13 is prime, but 14, which can be divided by two and seven, is not. The ancient Greeks knew that there are an infinite number of primes, but no one has come up with a formula for generating

them. The latest prime has been discovered recently on a Cray X-MP super-computer that was being tested by Chevron Geosciences Company in Houston, the “Times” said. The machine, which cost more than SUSIO million, was recently delivered to Chevron, which planned to use it to analyse geological data in exploring for oil.

To test the machine Chevron computer scientists, under the direction of a vice-president, William Bartz, had run a special program that checked large numbers to determine if they were so-called Mersenne primes. The number they had discovered was the 30th Mersenne prime discovered. It

had taken more than three hours to test the number on a machine that did 400 million calculations a second, the “Time” report said. Mersenne primes were named for Marin Mersenne, a 17th century French monk who investigated them. They take the form 2 raised to a prime power minus 1. The first three Mersenne primes are 3 (2 to the 2nd power minus 1), 7 (2 to the 3rd power minus 1) and 31 (2 to the sth power minus 1). “We just happened to crunch enough numbers to come up with a new prime,” Mr Bartz said. "It’s my responsibility to get the machine up and running and make sure we have a good one and not a lemon. “The results are interesting, if true, but they are

certainly not going to help me find oil,” he said. “Searching for such numbers is a learning experience, and it’s a challenge,” said Steve McGrogan, of Elxsi Computers, in San Jose, who doubled-checked the Houston work. “It’s like Mount Everest. Why do people climb mountains?”

While there is no known practical use for 65,000-digit prime numbers, the method that finds them requires trillions of calculations and is therefore a useful test of the reliability of large super-computers. It has also engendered an informal competition among supercomputer makers for recognition as the fastest machines on the market. The faster a computer is the larger the numbers it can test.