ABSTRACT
We shall consider invertible transformations of the (x, y) plane https://www.w3.org/1998/Math/MathML"> x ¯ = φ ( x , y , a ) , y ¯ = Ψ ( x , y , a ) , https://s3-euw1-ap-pe-df-pch-content-public-p.s3.eu-west-1.amazonaws.com/9781003419808/bdc5584f-066d-4a76-8427-d3482a92b96b/content/math1_1.tif" xmlns:xlink="https://www.w3.org/1999/xlink"/>
