This is the first-order Runge-Kutta formula, which is the same as the Euler method. It is the
most basic explicit method for numerical integration of ordinary differential equations
and is the simplest Runge-Kutta method.
D. Greenspan, Numerical Solution of Ordinary Differential Equations: for Classical,
Relativistic and Nano Systems, 1st ed, Wiley-VCH, 2006.