  The third law of thermodynamics in the form of the unattainability principle
states that exact ground-state cooling requires infinite resources. Here we
investigate the amount of non-equilibrium resources needed for approximate
cooling. We consider as resource any system out of equilibrium, allowing for
resources beyond the i.i.d. assumption and including the input of work as a
particular case. We establish in full generality a sufficient and a necessary
condition for cooling and show that for a vast class of non-equilibrium
resources these two conditions coincide, providing a single necessary and
sufficient criterion. Such conditions are expressed in terms of a single
function playing a similar role for the third law to the one of the free energy
for the second law. From a technical point of view we provide new results about
concavity/convexity of certain Renyi-divergences, which might be of independent
interest.
