Bifurcation of Periodic Orbit for a Class of Analytic Perturbation Systems

Consider the following analytic perturbation system (I)_λ x ˙ = y + λ f 1 ( x , y ) y ˙ = − x + λ f 2 ( x , y ) https://s3-euw1-ap-pe-df-pch-content-public-p.s3.eu-west-1.amazonaws.com/9781003067498/6ba0c26b-f086-40cc-86ed-1693dc596221/content/eq5193.tif"/>