  We study increasing stability in the inverse source problems for the
Helmholtz equation and the classical Lame system from (minimal) boundary data
at multiple wave numbers. By using the Fourier transform with respect to wave
numbers, explicit bounds for analytic continuation of the data to larger wave
numbers, the Hyugens' principle, and sharp bounds in the corresponding
dynamical initial boundary value problems, increasing (with growing wave
numbers interval) stability estimates for source terms are obtained.
