A simple function is a finite linear combination of indicator functions: φ = Σᵢ aᵢ 𝟙ₐᵢ. Every non-negative measurable function is the pointwise limit of an increasing sequence of simple functions. Simple functions form the foundation for constructing the Lebesgue integral.
Construct increasing sequences of simple approximations by discretizing height levels of a given measurable function.
Simple functions must be finite sums. While countable sums of measurable functions remain measurable, they are no longer 'simple.' Approximation is pointwise, not uniform.
Before the Lebesgue integral can be defined for arbitrary measurable functions, we need a class of functions simple enough to integrate by inspection yet rich enough to approximate anything. Simple functions fill this role exactly.
A simple function is a finite linear combination of indicator functions: φ = a₁𝟙_{A₁} + a₂𝟙_{A₂} + ... + aₙ𝟙_{Aₙ}, where each Aᵢ is a measurable set and each aᵢ is a real number. The indicator 𝟙_A equals 1 on A and 0 outside it, so φ is a function taking only finitely many values, each on a measurable set. Think of a histogram with flat horizontal bars: that picture is a simple function. Its integral is immediate — ∫φ dμ = Σ aᵢμ(Aᵢ) — a weighted sum of the measures of its level sets.
The central theorem is that every non-negative measurable function can be approximated from below by an increasing sequence of simple functions. The construction is geometric: divide the "height axis" into strips of width 1/n. For each integer k from 1 to n², define the set where k/n ≤ f(x) < (k+1)/n, and assign height k/n there. Stack these indicator functions to build a staircase φₙ that increases pointwise toward f. As n → ∞, the stairs become infinitely fine and φₙ(x) → f(x) at every point. The sequence {φₙ} is monotone increasing and converges to f pointwise everywhere.
This approximation theorem is the engine of the Lebesgue integral. For a general non-negative measurable function, the integral is defined as ∫f dμ = lim_{n→∞} ∫φₙ dμ — you integrate the simple approximations and take the limit. The theorem guarantees this bridge exists for every non-negative measurable function, not just well-behaved ones. The requirement that the sum defining a simple function is *finite* is essential: infinite sums would reintroduce the convergence problems the theory is designed to handle, and the clean formula ∫φ dμ = Σ aᵢμ(Aᵢ) depends on having only finitely many terms to sum.