Separability of pure n-qubit states: two characterizations

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.