function yp = vanderpol ( x, y )

%*****************************************************************************80
%
%% vanderpol() evaluates the right hand side of a vanderpol ODE.
%
%  Licensing:
%
%    This code is distributed under the MIT license.
%
%  Modified:
%
%    20 September 2019
%
%  Author:
%
%    John Burkardt
%
%  Input:
%
%    real x: the value of the independent variable.
%
%    real y(2): the value of the dependent variables.
%
%  Output:
%
%    real yp(2): the right hand side of the differential equation.
%
  global mu

  yp = zeros ( 2, 1 );
  u = y(1);
  v = y(2);

  yp(1) = v;
  yp(2) = - u + mu * ( 1.0 - u.^2 ) * v;

  return
end

