Phenomenological studies on the proton structure function F^2 have been carried out to investigate the behavior of F^2, i.e. its x and Q^2 dependence, in the transition from the deep-inelastic scattering region (Q^2 >> 1 GeV^2) to the photoproduction region (Q^2 approx. 0 GeV^2). An overview of various results of F^2 at low Q^2 is given. To quantify the behavior of F2 at low Q^2, the derivatives dln(F^2)/dln(x) for fixed Q^2 and dF^2/dlog(Q^2) for fixed x haven been determined and compared to expectations within the framework of perturbative and non-perturbative QCD. Furthermore, the derivatives were compared to results of the total inclusive diffractive cross-section and vector-meson production. An interpretation of inclusive ep results is given in the framework of the GVD/CDP picture with special emphasis on the limit of Q^2 --> 0 for fixed W^2 and x --> for fixed Q^2.
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