  The determination of an unknown spacewice dependent force function acting on
a vibrating string from over-specified Cauchy boundary data is investigated
numerically using the boundary element method (BEM) combined with a regularized
method of separating variables. This linear inverse problem is ill-posed since
small errors in the input data cause large errors in the output force solution.
Consequently, when the input data is contaminated with noise we use the
Tikhonov regularization method in order to obtain a stable solution. The choice
of the regularization parameter is based on the L-curve method. Numerical
results show that the solution is accurate for exact data and stable for noisy
data
