  We discuss properties of entanglement measures called I-concurrence and
tangle. For a bipartite pure state, I-concurrence and tangle are simply related
to the purity of the marginal density operators. The I-concurrence (tangle) of
a bipartite mixed state is the minimum average I-concurrence (tangle) of
ensemble decompositions of pure states of the joint density operator. Terhal
and Vollbrecht [Phys. Rev. Lett. 85, 2625 (2000)] have given an explicit
formula for the entanglement of formation of isotropic states in arbitrary
dimensions. We use their formalism to derive comparable expressions for the
I-concurrence and tangle of isotropic states.
