Solution
= Solution
A bipartite <pure state> is \b[entangled exactly when it is not a product state]: there are no normalized <quantum states> $|u\rangle_A,|v\rangle_B$ such that
$$
|\psi\rangle_{AB}=|u\rangle_A\otimes|v\rangle_B.
$$
Equivalently, its <Schmidt rank> exceeds one, or either <reduced density matrix> is a <mixed state>. This definition concerns <quantum entanglement> across the specified division into systems $A$ and $B$.