//! compute vector of derivatives of the least square function
public void gradient(ref Vector grad_f, Vector x)
{
// size of target and function to fit vectors
Vector target = new Vector(lsp_.size());
Vector fct2fit = new Vector(lsp_.size());
// size of gradient matrix
Matrix grad_fct2fit = new Matrix(lsp_.size(), x.size());
// compute its values
lsp_.targetValueAndGradient(x, ref grad_fct2fit, ref target, ref fct2fit);
// do the difference
Vector diff = target - fct2fit;
// compute derivative
grad_f = -2.0 * (Matrix.transpose(grad_fct2fit) * diff);
}
