function dmsedc = meanSquaredGradientCubic(x, y, c)
% The gradient of the mean squared error for a cubic relation.
%
% Arguments: x, vector of the x-data values
%            y, vector of the corresponding y-data values
%            c, the factor for the cubic relation. 
%
% Returns: the derivative of the mean squared error at c.

  h = 1e-7;   % stepsize for derivatives
  mse = meanSquaredErrorCubic(x, y, c);
  mseh = meanSquaredErrorCubic(x, y, c+h);
  dmsedc = (mseh - mse)/h;
end