English
If f is convex on s and g concave on s with f(x) ≤ 0 and g(x) ≤ 0 for x ∈ s, and MonovaryOn f g s holds, then the product h = f · g is concave on s.
Русский
Если f выпукла, g конкавна, f(x) ≤ 0 и g(x) ≤ 0 на s, и MonovaryOn(f,g,s) выполняется, то h=f g конкавна на s.
LaTeX
$$$\begin{aligned} &\text{ConvexOn}(s,f) \land \text{ConcaveOn}(s,g) \land \forall x\in s: f(x) \le 0, \ g(x) \le 0 \\ &\ &\text{MonovaryOn}(f,g,s) \Rightarrow \text{ConcaveOn}(s, f\cdot g). \end{aligned}$$$
Lean4
theorem mul_concaveOn' (hf : ConvexOn 𝕜 s f) (hg : ConcaveOn 𝕜 s g) (hf₀ : ∀ ⦃x⦄, x ∈ s → f x ≤ 0)
(hg₀ : ∀ ⦃x⦄, x ∈ s → 0 ≤ g x) (hfg : MonovaryOn f g s) : ConvexOn 𝕜 s (f * g) :=
hf.smul_concaveOn' hg hf₀ hg₀ hfg
