  We use Heegaard Floer homology to obtain bounds on unknotting numbers. This
is a generalisation of Ozsvath and Szabo's obstruction to unknotting number
one. We determine the unknotting numbers of 9_10, 9_13, 9_35, 9_38, 10_53,
10_101 and 10_120; this completes the table of unknotting numbers for prime
knots with crossing number nine or less. Our obstruction uses a refined version
of Montesinos' theorem which gives a Dehn surgery description of the branched
double cover of a knot.
