Periodic/Quasiperiodic Solutions of Second Order Duffing’s Equations

We are concerned with the following Duffing’s equation (1) x ¨ + g ( x ) = p ( t ) , https://s3-euw1-ap-pe-df-pch-content-public-p.s3.eu-west-1.amazonaws.com/9781003067498/6ba0c26b-f086-40cc-86ed-1693dc596221/content/eq506.tif"/>