To check out a new typewriter, one types "The quick brown fox . . . ." When the computer people at Chevron Geosciences Co. in Houston wanted to check out a new supercomputer intended to analyze geological data, they asked it to run a standard program that looks for prime numbers.

The machine, a $10 million Cray X-MP, obliged. After three hours of performing about 400 million calculations per second, it came up with the largest prime number yet discovered -- one that has about 65,000 digits. Expressed in the form that scientists favor, it is 2 raised to the 216,091st power minus 1.

Prime numbers, which are mostly of theoretical interest, are divisible only by themselves and 1. For example, 3 is a prime number but 4, because it can be divided evenly by 2, is not. The ancient Greeks knew there were an infinite number of prime numbers.

The engineers were not expecting to run the program long enough to find a new prime number, but the computer achieved the feat sooner than expected.

"We just happened to crunch enough numbers to come up with a new prime," said William Bartz, a Chevron vice president. "The results are interesting, if true, but they are certainly not going to help me find oil."