  We study a Dirichlet type problem for an equation involving the fractional
Laplacian and a reaction term subject to either subcritical or critical growth
conditions, depending on a positive parameter. Applying a critical point result
of Bonanno, we prove existence of one or two positive solutions as soon as the
parameter lies under a (explicitly determined) threshold. As an application, we
find two positive solutions for a fractional Brezis-Nirenberg problem.
