“…Other related papers on this type of inverse problem for pde's can be listed: [1,26] (one dimensional fourth order parabolic equation), [10,4,3] (discontinuous coefficients), [2] (network of one dimensional waves) [9,8] (logarithmic stability estimates), [21] (parabolic system), [41] (unknown coefficient in the nonlinearity), [46,15,21] (two unknown coefficients). We also mention some important books which can be the starting point to study inverse problems [30], Carleman estimates [22], and control theory for partial differential equations [18].…”