18th century -- classical mechanics -- ordinary differential equations.
19th century -- continuum mechanics -- partial differential equations.
20th century --- turbulence, plasticity--- ???
>>9068513
Integral equation
>https://en.wikipedia.org/wiki/Integral_equation
[math]\varphi (x)=f(x)+\lambda \int _{a}^{x}K(x,t)\,\varphi (t)\,dt[/math]