:For every ε > 0, there is a δ > 0 such that, whenever <math>0 < |x-X| < \delta, |f(x)-f(X)| < \varepsilon\,</math> | :For every ε > 0, there is a δ > 0 such that, whenever <math>0 < |x-X| < \delta, |f(x)-f(X)| < \varepsilon\,</math> |