  We consider entanglement in a system of fixed number of identical particles.
Since any operation should be symmetrized over all the identical particles and
there is the precondition that the spatial wave functions overlap, the meaning
of identical-particle entanglement is fundamentally different from that of
distinguishable particles. The identical-particle counterpart of the Schmidt
basis is shown to be the single-particle basis in which the one-particle
reduced density matrix is diagonal. But it does not play a special role in the
issue of entanglement, which depends on the single-particle basis chosen. The
nonfactorization due to (anti)symmetrization is naturally excluded by using the
(anti)symmetrized basis or, equivalently, the particle number representation.
The natural degrees of freedom in quantifying the identical-particle
entanglement in a chosen single-particle basis are occupation numbers of
different single particle basis states. The entanglement between effectively
distinguishable spins is shown to be a special case of the occupation-number
entanglement.
