Analytical formula connecting entangled states and the closest disentangled state

The separable state closest to a given entangled state in the relative entropy measure is called the closest disentangled state. We provide an analytical formula connecting the entangled states and the closest disentangled state in two qubits. Using this formula, when any disentangled state sigma located at the entangle-disentangle boundary is given, entangled states to which sigma is closest can be obtained analytically. Further, this formula naturally defines the direction normal to the boundary surface. The direction is uniquely determined by sigma in almost all cases.