The achievement of Dr John Cosgrave and Yves Gallot is a prime (pun intended) example of Net co-operation. Computers have always been good at the day-to-day calculations that we can all do but would rather not. But in the realm where the largest composite Fermat number and 10th-largest prime were found, computers themselves begin to choke on the sheer size of the calculations involved.

On the one hand, ever more powerful computers can be pushed into the fray. The record for a composite Fermat number broken by Cosgrave-Gallot was set at Silicon Graphics, the manufacturer of high-powered computer workstations.

Another approach, the one that succeeded in Dublin, is to distribute the work around the world, using the Internet for co-ordination. Researchers take on chunks of the numberspace and run Gallot's proth.exe program with as much computing power as they can muster to explore their area.

That was the background to Dr John Cosgrave leaving his office just before 5 p.m. on July 23rd to check over 40 PCs in the college's main computer lab on which he had been running Gallot's program for two months over the summer break. As he relates on his website: "I refreshed the screens of computer after computer, and clicked up the screens to view earlier computations. "In the third row, I examined computer 17, the second-last one in that row. After a few seconds I registered - with great delight - that the number 3 x 2 to-the-power-of 382449 +1 was a prime, and had only been registered as such some minutes earlier."

This led to a dilemma. Proth.exe performs a series of extra tests, for possible division into related Fermat numbers. Computer 17 reported that it was doing a further calculation, one that would take 1200 minutes and report on Saturday - the 28th wedding anniversary of Dr Cosgrave and his wife Mary. "Could I risk coming into college at such a time," he wrote "when we would be about to go out for a meal together? It had to be `No!' I could come in first thing on Monday morning. . ."

He had announced the discovery of the prime by email on Friday and on Sunday an excited reply arrived from Gallot, who advised him to check the follow-up calculation. The Cosgraves set out for a walk, but the excitement of discovery won out and instead they cycled across Dublin to the college. Waiting there was the record-breaking composite Fermat number.