ON THE SINGULAR BEHAVIOR OF STRUCTURE FUNCTIONS AT LOW X

We discuss the phenomenological extraction of the leading j-plane sing ularity from singlet structure functions F(s) estimated at small x. Us ing a saddle point method we show that dln F(s)/dln 1/x is a suitable observable for this purpose in the region x less-than-or-equal-to 10(- 2). As an application, we confront and distinguish in a model-independ ent way structure function parametrizations coming from two different QCD evolution equations, namely the Lipatov (or BFKL) equation and the Gribov-Lipatov-Altarelli-Parisi (or GLAP) equation. Recent results on the proton structure function F2 at HERA are discussed in this framew ork.