In an isotropic medium, Fourier's law leads to the heat equation ∂ ∂ T ∂ ∂ t = α α ( ∂ ∂ 2 T ∂ ∂ x 2 + ∂ ∂ 2 T ∂ ∂ y 2 + ∂ ∂ 2 T ∂ ∂ z 2 ) with a fundamental solution famously known as the heat kernel.