  In classical and quantum information theory, operational quantities such as
the amount of randomness that can be extracted from a given source or the
amount of space needed to store given data are normally characterized by one of
two entropy measures, called smooth min-entropy and smooth max-entropy,
respectively. While both entropies are equal to the von Neumann entropy in
certain special cases (e.g., asymptotically, for many independent repetitions
of the given data), their values can differ arbitrarily in the general case.
  In this work, a recently discovered duality relation between (non-smooth)
min- and max-entropies is extended to the smooth case. More precisely, it is
shown that the smooth min-entropy of a system A conditioned on a system B
equals the negative of the smooth max-entropy of A conditioned on a purifying
system C. This result immediately implies that certain operational quantities
(such as the amount of compression and the amount of randomness that can be
extracted from given data) are related. Such relations may, for example, have
applications in cryptographic security proofs.
