“…They occur in various practical applications, including acoustic tomography [3,7,13,15], medical imaging [2,4,10], pollution of environment [8], gesture-computing technology [11,14,16] and have been vigorously studied in recent years. In the fixed frequency case, the inverse source problems possess non-unique solutions due to the presence of a non-radiating source [1,6,12,17]. One of the ways to overcome this difficulty consists in the use of multi-frequency data [5].…”