A recent problem in
dynamics is to determine whether an attractor Λ of a Cr flow X is Cr robust
transitive. By an attractor we mean a transitive set to which all positive orbits close
to it converge. An attractor is Cr robust transitive (or Crrobust for short) if it has a
neighborhood U such that the set ⋂t>0Yt(U) is transitive for every flow Y that is Cr
close to X. We give sufficient conditions for robustness of attractors based on the
following definitions: an attractor is singular-hyperbolic if it has singularities, all of
which are hyperbolic, and is partially hyperbolic with volume expanding central
direction (Morales, Pacifico and Pujals, 1998). An attractor is Crcritically robust if
it has a neighborhood U such that ⋂t>0Yt(U) is in the closure of the closed orbits of
every flow YCr close to X. We show that on compact 3-manifolds all Cr
critically robust singular-hyperbolic attractors with only one singularity are Cr
robust.