  Wootters [PRL 80, 2245 (1998)] has derived a closed formula for the
entanglement of formation (EOF) of an arbitrary mixed state in a system of two
qubits. There is no known closed form expression for the EOF of an arbitrary
mixed state in any system more complicated than two qubits. This paper, via a
relatively straightforward generalization of Wootters' original derivation,
obtains a closed form lower bound on the EOF of an arbitary mixed state of a
system composed of a qubit and a qudit (a d-level quantum system, with d
greater than or equal to 3). The derivation of the lower bound is detailed for
a system composed of a qubit and a qutrit (d = 3); the generalization to d
greater than 3 then follows readily.
