  Wigner and Yanase introduced in 1963 the Wigner-Yanase entropy defined as
minus the skew information of a state with respect to a conserved observable.
They proved that the Wigner-Yanase entropy is a concave function in the state
and conjectured that it is subadditive with respect to the aggregation of
possibly interacting subsystems. While this turned out to be true for the
quantum-mechanical entropy, we negate the conjecture for the Wigner-Yanase
entropy by providing a counter example.
