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For a number years (up to the Sept. 2008) GIMPS / PrimeNet limited the range of exponents that were available for assignment/testing to a maximum of ~79.3 million. This seemingly odd number is derived from the fact that it is the largest to practically test using an FFT size of 4096K. The size limit of the FFT used came about because of limitations of the computer systems that Prime95 was designed to run on.
Exponents larger than 79.3 had been worked on (mainly trial factoring, like the 100 Million Digit Prefactor Project) prior to Sept. 2008 (outside of PrimeNet), using programs other than Prime95. The results of these test were often forward to George Woltman and Will Edgington. When PrimeNet v5 was rolled out, exponents up to 1,000,000,000 were suppported. All of the work that had been previously reported to Woltman, was include in the data on PrimeNet.