1. A D Martin, R G Roberts, W J Stirling and R S Thorne, Europhys. J. C4, 463 (1998)
2. E Gotsman, E Levin, U Maor and E Naftali, Nucl. Phys. B539, 535 (1999)
3. A D Martin, M G Ryskin and A M Stasto, Europhys. J. C7, 643 (1999)
4. R Basu and D Indumathi, Phys. Lett. B452, 115 (1999)
5. The ZEUS Collab.: Europhys. J. C7, 609 (1999)