  We show that the Smoluchowski coagulation equation with the solvable kernels
$K(x,y)$ equal to $2$, $x+y$ or $xy$ is contractive in suitable Laplace norms.
In particular, this proves exponential convergence to a self-similar profile in
these norms. These results are parallel to similar properties of Maxwell models
for Boltzmann-type equations, and extend already existing results on
exponential convergence to self-similarity for Smoluchowski's coagulation
equation.
