What’s mass estimated in pde ?
The example we can integrate by mass estimated on space.
For $u_t(x, t)=u_{xx}(x, t)$ By maximum principle we have $u\le M$ aan nd $v\le N$.
So we can integrate $u_t$ on space say (0,1) for an easy choice.
That’s $\int^1_0u_t dx$ By mass estimated.
Some author say easy to see by mass estimated, that my question how use the mass estimated.