Let
be a differentiable function such that f ′ ( x ) = 0 {\displaystyle {}f'(x)=0} for all x ∈ ] a , b [ {\displaystyle {}x\in {]a,b[}} .
![{\displaystyle f\colon {]a,b[}\longrightarrow \mathbb {R} }](https://wikimedia.org/api/rest_v1/media/math/render/svg/7bcaf3655764f40916a38f2b7b57dd1653da0509)
for all
![{\displaystyle {}x\in {]a,b[}}](https://wikimedia.org/api/rest_v1/media/math/render/svg/50d5258fad15c7c5b7180a8778c8e887d86864ef)
.
