# Van der Pol equation

The non-linear second-order ordinary differential equation

(1) |

which is an important special case of the Liénard equation. Van der Pol's equation describes the auto-oscillations (cf. Auto-oscillation) of one of the simplest oscillating systems (the van der Pol oscillator). In particular, equation (1) serves — after making several simplifying assumptions — as a mathematical model of a generator on a triode for a tube with a cubic characteristic. The character of the solutions of equation (1) was first studied in detail by B. van der Pol .

Equation (1) is equivalent to the following system of two equations in two phase variables :

(2) |

It is sometimes convenient to replace the variable by the variable ; equation (1) then becomes

which is a special case of the Rayleigh equation. If, together with , one also considers the variable , introduces a new time and puts , one obtains the system

(3) |

instead of equation (1). For any there exists a unique stable limit cycle in the phase plane of the system (2) to which all other trajectories (except for the equilibrium position at the coordinate origin) converge as ; this limit cycle describes the oscillations of the van der Pol oscillator [2], [3], [4].

For small the auto-oscillations of the oscillator (1) are close to simple harmonic oscillations (cf. Non-linear oscillations) with period and specified amplitude. In order to calculate the oscillation process more accurately, asymptotic methods are employed. As increases, the auto-oscillations of the oscillator (1) deviate more and more from harmonic oscillations. If is large, equation (1) describes relaxation oscillation with period (to a first approximation). More accurate asymptotic expansions of magnitudes characterizing relaxation oscillations [5] are known: The study of these oscillations is equivalent to the study of the solutions of the system (3) with a small coefficient in front of the derivative [6].

The equation

describes the behaviour of the van der Pol oscillator when acted upon by a periodic external disturbance. The most important in this context is the study of frequency capture (the existence of periodic oscillations), beats (the possibility of almost-periodic oscillations) and chaotic behaviour [2], [4].

#### References

[1a] | B. van der Pol, "On oscillation hysteresis in a triode generator with two degrees of freedom" Philos. Mag. (6) , 43 (1922) pp. 700–719 |

[1b] | B. van der Pol, Philos. Mag. (7) , 2 (1926) pp. 978–992 |

[2] | A.A. Andronov, A.A. Vitt, A.E. Khaikin, "Theory of oscillators" , Dover, reprint (1987) (Translated from Russian) |

[3] | S. Lefschetz, "Differential equations: geometric theory" , Interscience (1957) |

[4] | J.J. Stoker, "Nonlinear vibrations in mechanical and electrical systems" , Interscience (1950) |

[5] | A.A. Dorodnitsyn, "Asymptotic solution of the van der Pol equation" Priklad. Mat. Mekh. , 11 (1947) pp. 313–328 (In Russian) (English abstract) |

[6] | E.F. Mishchenko, N.Kh. Rozov, "Differential equations with small parameters and relaxation oscillations" , Plenum (1980) (Translated from Russian) |

#### Comments

For small the first terms of the series for amplitude and period have been computed by symbolic calculation, see [a1]. The computation of [5] has been refined in [a2]. For a recent survey of the free and forced van der Pol oscillator, see [a3].

#### References

[a1] | M.B. Dadfar, J. Geer, C.M. Andersen, "Perturbation analysis of the limit cycle of the free Van der Pol equation" SIAM J. Appl. Math. , 44 (1984) pp. 881–895 |

[a2] | H. Bavinck, J. Grasman, "The method of matched asymptotic expansions for the periodic solution of the Van der Pol equation" Int. J. Nonlin. Mech. , 9 (1974) pp. 421–434 |

[a3] | J. Grasman, "Asymptotic methods for relaxation oscillations and applications" , Springer (1987) |

[a4] | J. Guckenheimer, P. Holmes, "Non-linear oscillations, dynamical systems, and bifurcations of vector fields" , Springer (1983) |

**How to Cite This Entry:**

Van der Pol equation.

*Encyclopedia of Mathematics.*URL: http://encyclopediaofmath.org/index.php?title=Van_der_Pol_equation&oldid=12868