  A Lie-algebra based recipe for smoothing gauge links in lattice field theory
is presented, building on the matrix logarithm. With or without hypercubic
nesting, this LOG/HYL smearing yields fat links which are differentiable w.r.t.
the original ones. This is essential for defining UV-filtered ("fat link")
fermion actions which may be simulated with a HMC-type algorithm. The effect of
this smearing on the distribution of plaquettes and on the residual mass of
tree-level O(a)-improved clover fermions in quenched QCD is studied.
