Motivated by the question of existence of global solutions, we obtain pointwise upper
bounds for radially symmetric and monotone solutions to the homogeneous Landau
equation with Coulomb potential. The estimates say that blow-up in the
norm at some finite time
occurs only if a certain
quotient involving
and its Newtonian potential concentrates near zero, which implies blow-up in more standard
norms, such as the
norm. This quotient is shown to be always less than a universal constant, suggesting
that the problem of regularity for the Landau equation is in some sense
critical.
The bounds are obtained using the comparison principle both for the
Landau equation and for the associated mass function. In particular, the
method provides long-time existence results for a modified version of the
Landau equation with Coulomb potential, recently introduced by Krieger and
Strain.