Generalized Periodic Solutions of f(t)2 + g(t)2 = 1

The equation (5.1) f 2 ( t ) + g 2 ( t ) = 1 https://s3-euw1-ap-pe-df-pch-content-public-p.s3.eu-west-1.amazonaws.com/9780429446238/d61a7316-5f65-4e0a-956e-ef5c7afc82a1/content/eq441.tif"/>