function value = ff ( x )

%% FF evaluates the function F in the differential equation.
%
%  Discussion:
%
%    This is the function F(X) that appears on the right hand
%    side of the equation:
%
%      -d/dx ( P(x) * dU(x)/dx ) + Q(x) * U(x)  =  F(x)
%
%  Modified:
%
%    05 November 2006
%
%  Parameters:
%
%    Input, real X, the evaluation point.
%
%    Output, real VALUE, the value of F(X).
%

%
%  Find out which problem we're working on.
%
  problem = get_problem ( 'DUMMY' );

  if ( problem == 1 )

    value = 0.0;

  elseif ( problem == 2 )

    value = -2.0 * x;

  elseif ( problem == 3 )

    value = 0.25 * pi^2 * sin ( 0.5 * pi * x );

  elseif ( problem == 4 )

    value = 0.25 * pi^2 * cos ( 0.5 * pi * x );

  elseif ( problem == 5 )

    beta = get_beta ( 'DUMMY' );

    value = - ( x^beta ) + ( x^( beta + 2.0 ) ) ...
      / ( ( beta + 2.0 ) * ( beta + 1.0 ) );

  elseif ( problem == 6 )

    alpha = get_alpha ( 'DUMMY' );

    value = 2.0 * alpha * ( x - 0.5 ) ...
      / ( alpha^2 + ( x - 0.5 )^2 )^2;

  end