LEAD PARAGRAPH - A Japanese laboratory has built the world's fastest computer, a machine so powerful that it matches the raw processing power of the 20 fastest American computers combined and far outstrips the previous leader, an I.B.M.-built machine.

The achievement, which was reported today by an American scientist who tracks the performance of the world's most powerful computers, is evidence that a technology race that most American engineers thought they were winning handily is far from over. American companies have built the fastest computers for most of the last decade.

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