“…. By substituting this linear solution into (18), the coefficients of the second-order solution can be solved using the Lindstedt-Poincaré method. Similarly, when the coefficients up to order N − 1 are obtained, i.e., x(t), y(t), and z(t) are determined up to order N − 1, ω and ∆ are determined up to order N − 2.…”