总结：
1、为了更加便于求解，将方程从原域映射到另外一个域
Integral transform - Wikipedia https://en.wikipedia.org/wiki/Integral_transform
In mathematics, an integral transform maps an equation from its original domain into another domain where it might be manipulated and solved much more easily than in the original domain. The solution is then mapped back to the original domain using the inverse of the integral transform.
积变换表 Table of integral transforms
