linguistic:paraphrasing, structural:scholar_edition
Differential wave equations

where Q (p, f) {\displaystyle Q(p,f)} is the heat that is being generated per unit of volume and time in the neighborhood of x {\displaystyle x} at time t {\displaystyle t} (for example, by chemical reactions happening there); x 1, x 2, x 3 {\displaystyle x_{1},x_{2},x_{3}} are the Cartesian coordinates of the point x {\displaystyle x}; ∂ F / ∂ t {\displaystyle \partial F/\partial t} is the (first) derivative of F {\displaystyle F} with respect to t {\displaystyle t}; and ∂ 2 F / ∂ x i 2 {\displaystyle \partial ^{2}F/\partial x_{i}^{2}} is the second derivative of F {\displaystyle F} relative to x i {\displaystyle x_{i}}. (The symbol " ∂ {\displaystyle \partial } " is meant to signify that, in the derivative with respect to some variable, all other variables must be considered fixed.)