The van der Pol equation is anthat can be derived from the by differentiating and setting . It is an equation describing self-sustaining oscillations in which energy is fed into small oscillations and removed from large oscillations. This equation arises in the study of circuits containing vacuum tubes and is given by
If , the equation reduces to the equation of
The van der Pol equation is
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
Any convenient numerical differential equation solver such as the Runge-Kutta method can be used to compute the solutions.