What is the mathematical expression for a function to be strictly increasing on (a,b)?

Options :

1. x₁ < x₂ ⇒ f(x₁) < f(x₂) ∀ x₁, x₂ ∈ (a,b)

2. x₁ < x₂ ⇒ f(x₁) ≥ f(x₂) ∀ x₁, x₂ ∈ (a,b)

3. x₁ = x₂ ⇒ f(x₁) ≤ f(x₂) ∀ x₁, x₂ ∈ (a,b)

4. x₁ = x₂ ⇒ f(x₁) < f(x₂) ∀ x₁, x₂ ∈ (a,b)