Applied Mathematics of Interconnected Phenomena
Journal license

Journal

Applied Mathematics of Interconnected Phenomena


Volume
& Issue

Volume 1, Issue 1


Published
on

October 30, 2026


Pages

1-15


DOI

Article

Parametric Entrainment and Amplitude Enhancement in a Van der Pol Oscillator with State-Multiplicative Periodic Forcing: A Rigorous Averaging Analysis

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Authors

Adrián Valverde Mateo* Affiliation:
Departamento de Ingeniería y Tecnología de Computadores, Facultad de Informática, Campus de Espinardo, Universidad de Murcia, Spain


Abstract

We study the Van der Pol oscillator \(\ddot{x} - \mu(1-x^2)\dot{x} + x = Fx\cos\omega t\), in which the external excitation is multiplied by the state, so that the forcing acts parametrically and vanishes at the origin. Written as a Van der Pol–Mathieu oscillator, the equation admits a complete analysis of its 2:1 resonance, and every result below is proved. First, the trivial solution is unstable for every \(F\) and \(\omega\) when \(\mu \gt 0\), with Floquet exponents \(\mu/2 \pm s\), where \(s\) is the exponent of an associated Mathieu equation; the sum of the exponents of any periodic orbit equals \(\mu(1-\langle x^2\rangle)\), and every periodic orbit satisfies an exact power balance. Second, the first-order averaged flow is triangular, and the substitution \(u=a^{-2}\) makes its amplitude equation linear; this yields global statements. Inside the tongue \(|\omega-2| \lt F/2\) all solutions except one are attracted to a subharmonic orbit of amplitude \(a_+ = 2\sqrt{1+\mathscr{D}/(2\mu)} \gt 2\), \(\mathscr{D} = \sqrt{F^2-4(\omega-2)^2}\), while a saddle orbit of amplitude \(a_- = 2\sqrt{1-\mathscr{D}/(2\mu)}\) exists if and only if \(\mathscr{D} \lt 2\mu\); both merge with the origin on the hyperbola \(F^2 = 4(\omega-2)^2+4\mu^2\). Outside the tongue a unique attracting orbit of the slow flow, with closed-form amplitude envelope, transverse rate \(\mu\) and phase-rotation frequency \(\kappa/2\), is obtained. Third, by averaging and normal hyperbolicity these objects persist in the full equation as a hyperbolic subharmonic periodic orbit, a saddle periodic orbit and an attracting invariant torus, respectively. Dimensional design rules follow. An appendix, clearly separated from the proofs, confirms the first-order predictions and, for strong forcing, indicates chaos; two precise open problems are formulated.


Keywords

Van der Pol oscillator, Van der Pol–Mathieu equation, parametric resonance, averaging method, Arnold tongue, Floquet


Citation

Valverde Mateo, A. (2026). Parametric entrainment and amplitude enhancement in a van der pol oscillator with state-multiplicative periodic forcing: A rigorous averaging analysis. Applied Mathematics of Interconnected Phenomena, 1(1), 1–15. https://doi.org/10.66833/AMIP-2026-0002

Published by: Engineering Journals

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