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Math and science::Analysis::Tao::10: Differentiation of functions

Differentiability on a domain

Let \( X \) be a subset of \( \mathbb{R} \) and let \( f : X \to \mathbb{R} \) be a function. We say that \( f \) is differentiable on \( X \) iff [...].

Tao describes this as an if rather than iff statement.