Interlacing, integrals of Hurwitz series and differential equations

In this paper, we first introduce the unlacing of Hurwitz series, which can be viewed as an inverse of interlacing, and develop the basic properties of unlacing, interlacing and integral of Hurwitz series. We then show that the multiplication of interlacing of Hurwitz series by basis elements can be also rewritten as an interlacing, and provide the explicit multiplication formula in terms of integrals and Hadamard products. Finally, we investigate the natural exponential function $\exp$, and realize the method of reduction of order in the ring of Hurwitz series by using $\exp$.