ABSTRACT
The Laplacian in rectangular coordinates is Δ u = u x x + u y y + u z z $$ \Delta u = u_{{xx}} + u_{{yy}} + u_{{zz}} $$ https://s3-euw1-ap-pe-df-pch-content-public-p.s3.eu-west-1.amazonaws.com/9781315183138/c768726a-487f-48b7-9265-5d998f5a19e7/content/math14_1.tif"/>
