# Evans' proof of parabolic strong maximum principle

I'm reading his PDE in chapter 7.1 the last theorem is about the strong maximum principle for parabolic equation when $c\geq 0$ at page 398. I have some problem at the second step:

Since $u_t +Ku=-cu\leq 0$ on $\{ u\geq 0 \}$, we deduce from the weak maximum principle that $u\leq v$.

Who can give some detail to show how the weak maximum principle applies? Thanks in advance!

On the one hand, $$u_t + K u \leq 0$$ on $\{ u \geq 0\}$. On the other hand, $$v_t + K v = 0,$$ with $v = u^+$ on $\Delta_T$.
If we subtract second equation from the first one, we get $$(u-v)_t + K(u-v) \leq 0 \quad \implies \quad w_t + K w \leq 0,$$ where $w := u-v$. (We can do that, since $\frac{d}{dt}$ and $K$ are linear operators). Moreover, $w = 0$ on $\Delta_T$.
Hence, applying Weak Maximum Principle for $w$ (Theorem 8), we obtain $$\max_{W_T} w = \max_{\Delta_T} w = 0,$$ and therefore, $w \leq 0$. Thus, $u \leq v$.
• @Ylath Note that, due to the nonnegative boundary conditions, by Weak maximum principle $0 \leq v$ in $W_T$. If $u < 0$ somewhere in $W_T$, then $u < 0 \leq v$ automatically. – Voliar Jan 20 '15 at 5:33