Calderón–Zygmund Decomposition

A decomposition of an integrable function f on Euclidean space at a chosen level λ > 0 into f = g + b where g is a 'good' function with controlled L^∞ norm (bounded by a multiple of λ) and b is a sum of 'bad' functions b_j supported on disjoint cubes with mean zero and controlled L^1 mass; used to handle singular integral operators and prove weak-type and L^p bounds.