“…Using Duhamel's principle, an approach similar to Kleefeld et al and Yousuf et al , we can show that satisfy the following recurrent formula: The integral in (2.6) can be approximated by a class of exponential time differencing (ETD) numerical schemes, see for example and references therein. Denoting the approximation to by and the approximation to by , the second order exponential time differencing Runge–Kutta semidiscrete scheme is given by , …”