van der Pol System

The van der Pol Equation

    The van der Pol equation is

        
,  
where  
        
  is a constant.  
        
When  
  the equation reduces to  
,   and has the familiar solution  
.  Usually the term  
  in equation (1) should be regarded as friction or resistance, and this is the case when the coefficient
  is positive.  However, if the coefficient  
  is negative then we have the case of "negative resistance."  In the age of "vacuum tube" radios, the "tetrode vacuum tube" (cathode, grid, plate),  was used for a power amplifier and was known to exhibit "negative resistance."  The mathematics is amazing too, and  van der Pol, Balthasar (1889-1959) is credited with developing equation (1).  The solution curves exhibits orbital stability.

    The van der Pol equation can be written as a second order system  

    
,   
and  
    
.    

Any convenient numerical differential equation solver such as the Runge-Kutta method can be used to compute the solutions.  

Background. The Runge-Kutta method is used to numerically solve O.D.E.'s over
.

Extension to 2D. The Runge-Kutta method is easily extended to solve a system of D.E.'s over the interval  
.

Program (Runge-Kutta Method in 2D space)  To compute a numerical approximation for the solution of the initial value problem  

    
  with  
,  
    
  with  
,  

over the interval  
  at a discrete set of points.

Note.  The Runge-Kutta method in 2D is a "vector form" of the one-dimensional method, here the function f is replaced with F.