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    Soft and hard pomerons

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    Regge theory provides an excellent description of small-xx structure-function data from Q2=0Q^2=0 up to the highest available values. The large-Q2Q^2 data should also be described by perturbative QCD: the two descriptions must agree in the region where they overlap. However, at present there is a serious lack in our understanding of how to apply perturbative QCD at small xx. The usual lowest-order or next-to-lowest order expansion is not valid, at least not until Q2Q^2 becomes much larger than is usually assumed; a resummation is necessary, but as yet we do not know how to do this resummation.Comment: 8 pages, plain tex, with 6 figures embedded using epsf. Lecture at Workshop on New Trends in HERA Physics, Ringberg (june 1999
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