An identity is true regardless of the values of its variables. 恒等式中的变量变化不影响恒等式的成立。
After that, people found the identity has many different applications, especially on regularity and non-existence of solutions about PDEs. 自此以后，人们发现该恒等式有很多用途，尤其是在研究偏微分方程中解的存在性和唯一性方面。
