  This paper characterizes two forms of separability of pure states of systems
of n qubits: (i) into a tensor product of n qubit states, and (ii), into a
tensor product of 2 subsystems states of p and q qubits respectively with
p+q=n. For both cases, necessary and sufficient conditions are proved in the
form of minimal sets of equalities among pair-wise products of amplitudes.
These conditions bear some relation with entanglement measures, and a number of
more refined questions about separability in n qubit systems can be studied on
the basis of these results.
