ABSTRACT
When the temperature in a solid varies with time, it obeys the equation ∂ T ∂ τ = α ∇ 2 T , $$ \frac{{\partial T}}{{\partial \tau }} = \alpha \nabla ^{2} T, $$ https://s3-euw1-ap-pe-df-pch-content-public-p.s3.eu-west-1.amazonaws.com/9781315108858/f4dff3a2-1feb-457b-a709-022257f02721/content/math19_1.tif"/>
