  We study the effect of superconducting fluctuations on the upper critical
field of a disordered superconducting film at low temperatures. The first order
fluctuation correction is found explicitly. In the framework of the
perturbative analysis, superconducting fluctuations are shown to shift the
upper critical field line toward lower fields and do not lead to an upward
curvature. Higher order corrections to the quadratic term coefficient in the
Ginzburg-Landau free energy functional are studied. We extract a family of the
mostly divergent diagrams and formulate a general rule of calculating a diagram
of an arbitrary order. We find that the singularity gets more severe with
increasing perturbation theory order. We conclude that the renormalization of
the Ginzburg-Landau coefficients by superconducting fluctuations is an
essentially non-perturbative effect. As a result, the genuine transition line
may be strongly shifted from the classical mean-field curve in a
two-dimensional superconductor.
