In this paper, we investigate the existence and multiplicity of harmonic and subharmonic solutions for second-order quasilinear equationwhere p ∶ R → R, p (u) = |u| p−2 u, p > 1, g satisfies the superlinear condition at infinity. We prove that the given equation possesses harmonic and subharmonic solutions by using the phase-plane analysis methods and a generalized version of the Poincaré-Birkhoff twist theorem.
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CHENG AND RENNote that when p = 2, the differential operator x → ( p (x ′ )) ′ reduces to the linear operator x → x ′′ and then Equation 1 is of the Duffing equation formx ′′ + g(x) = e(t).(2)The periodic solution problem for the Duffing equation has attracted much attention, see, eg, previous studies. [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16] However, the study on quasilinear equation is relatively infrequent. The main difficulty lies in the quasilinear operator ( p (x ′ )) ′ that typically possesses more uncertainty than the linear operator x ′′ . In recent years, there also appears some results on second-order quasilinear equation, see other works. [17][18][19][20][21][22] In literature, 18 del Pino, Elgueta, and Manásevich considered the Dirichlet boundary value problemand the corresponding eigenvalue problemon a set of positive measure, then (3) has a nontrivial solution. Afterwards, del Pino, Manásevich, and Murúa 20 deal with T-periodic solutions for the nonlinear ordinary differential Equation 3, and f ∶ R × R → R is continuous and T-periodic in t, T > 0. When the nonlinearity f interacts with the Fučík spectrum for the operator u → (|u ′ | p−2 u ′ ) ′ , existence and multiplicity results are proved for (3). In a study, 21 Liu established some existence and multiplicity results for periodic solutions of the equationby applying the Poincaré-Birkhoff fixed-point theorem, where s ∈ R is a parameter, (t) is a continuous periodic function, and g(x) is locally Lipschitz on R and satisfies some asymptotic conditions. According to the growth speed of g, (1) can be classified into the following 3 cases,Lemma 2.1. (The Poincaré-Birkhoff twist theorem (see Theorem 1.2 of Fonda and Ureña 23 )) Under the above assumptions, the HS has at least 2 distinct mT-periodic solutions z (0) (t), z (1) (t), starting with z (k) (0) ∈ , such that Rot(z (k) (t); [0, mT]) = , for every k = 0, 1.Moreover, if H is twice continuously differentiable with respect to z and all mT-periodic solutions with initial condition on are nondegenerate, then there are at least 2 of them.Lemma 2.1 generalized some previous versions of the Poincaré-Birkhoff Theorem for planar annuli with star-shaped boundaries. [24][25][26] Consider the equivalent HS of (1), x ′ = q (y), y ′ = −g(x) + e(t),corresponding to the Hamiltonian H(t, x, y