PERIODIC SOLUTIONS OF A NON-SMALL AMPLITUDE OF THE QUASILINEAR EQUATION FOR OSCILLATIONS OF AN I-BEAM

Abstract

The problem of time-periodic solutions of the quasilinear equation of forced oscillations of an I-beam with hinged ends is investigated. The nonlinear summand and the right side of the equation are time periodic functions. The paper studies the case when the time period is commensurate with the length of the beam. The solution is sought in the form of a Fourier series. To prove the convergence of the Fourier series and their derivatives, we study the eigenvalues of the differential operator corresponding to the linear part of the equation. Conditions are obtained under which the kernel of a differential operator is finite-dimensional and the inverse operator is completely continuous on the complement of the kernel. A lemma on the existence and regularity of solutions of the corresponding linear problem is proved. The proof is based on the properties of the sums of the Fourier series. A theorem on the existence and regularity of a periodic solution is proved if the nonlinear term satisfies the non-resonance condition at infinity. The non-resonance condition implies the fact that, for large values of the argument, the graph of the non-linear term does not intersect the straight lines whose slope is an eigenvalue of the linear part of the equation. In the proof of the theorem, an a priori estimate is made of the solutions of the corresponding operator equation and the Leray-Schauder principle of a fixed point is applied. Additional conditions are obtained under which the periodic solution found in the main theorem is unique.

Keywords

• Beam oscillations
• periodic solutions
• Fourier series
• fixed points