“…The factorization method has first been introduced by Kirsch ( [16]) for the inverse acoustic obstacle scattering. It has been studied so far by many authors (see e.g., [1,3,4,7,11,17,18,19,20,22,24]), and it is well known as one of classical qualitative methods, which includes the linear sampling method of Colton and Kirsch ([5]), the singular sources method of Potthast ([23]), the probe method of Ikehata ([15]), etc. The monotonicity method, on the other hand, has been recently introduced by Harrach in [14] for the electrical impedance tomography.…”