ABSTRACT
By a second-order multiplicative partial differential equation in n multiplicative variables https://www.w3.org/1998/Math/MathML"> x 1 , https://s3-euw1-ap-pe-df-pch-content-public-p.s3.eu-west-1.amazonaws.com/9781003440116/5c34fd59-3787-4510-b376-2cd023dd9bdb/content/math2_1.tif" xmlns:xlink="https://www.w3.org/1999/xlink"/> https://www.w3.org/1998/Math/MathML"> … , https://s3-euw1-ap-pe-df-pch-content-public-p.s3.eu-west-1.amazonaws.com/9781003440116/5c34fd59-3787-4510-b376-2cd023dd9bdb/content/math2_2.tif" xmlns:xlink="https://www.w3.org/1999/xlink"/> x n , we mean any equation of the form https://www.w3.org/1998/Math/MathML"> F ( x 1 , … , x n , u , u x 1 ∗ , … , u x n ∗ , u x 1 x 1 ∗ ∗ , u x 1 x 2 ∗ ∗ , … , u x n x n ∗ ∗ ) = 0 ∗ https://s3-euw1-ap-pe-df-pch-content-public-p.s3.eu-west-1.amazonaws.com/9781003440116/5c34fd59-3787-4510-b376-2cd023dd9bdb/content/math2_3.tif" xmlns:xlink="https://www.w3.org/1999/xlink"/>
