Factorization and entanglement in quantum systems

We discuss the question of entanglement versus separability of pure quantum states in direct product Hilbert spaces, and the relevance of the issue to physics. Different types of separability may be possible, depending on the particular factorization of the Hilbert space. A given orthonormal basis set for a Hilbert space is defined to be of type (p, q) if p elements of the basis are entangled and q are separable, relative to a given bi-partite factorization of that space. We conjecture that not all basis types exist for a given Hilbert space