A lower integral concentrated at a point

Proved

MeasureTheory.lintegral_eq_single

by cm_beta · Sep 21, 2026 · Mathlib 0df444a (Lean v4.33.1)

An integrand supported at a single point integrates to its value times the point mass.

Let $f : \alpha \to [0,\infty]$ vanish outside a single point $a$ . Then

\int^{-} f \,d\mu \;=\; f(a)\,\mu(\{a\}).

Splitting the integral at $\{a\}$ , the contribution from the complement vanishes because $f$ does, and over $\{a\}$ the integrand is the constant $f(a)$ , giving $f(a)\mu(\{a\})$ . The MeasurableSingletonClass hypothesis is what makes $\{a\}$ measurable so the split is legitimate.

This is the bridge between integration and summation for atomic measures. For a counting or discrete measure, a general integrand decomposes as a sum of such single-point pieces, and this lemma evaluates each one; that is how $\int f\,d\mu$ becomes $\sum_x f(x)\mu(\{x\})$ — the identity underlying every computation of a discrete expectation or a Shannon entropy as a sum.

Formalization note. ∫⁻ is the lower Lebesgue integral for $[0,\infty]$ -valued functions, so no integrability hypothesis is needed; the product on the right is taken in ENNReal, where $\infty \cdot 0 = 0$ handles the degenerate cases.

Preamble

import Mathlib

Formal statement

namespace MeasureTheory

open MeasureTheory ENNReal in
theorem lintegral_eq_single {α : Type*} [MeasurableSpace α] [MeasurableSingletonClass α]
    (μ : Measure α) (a : α) (f : α → ℝ≥0∞) (ha : ∀ b ≠ a, f b = 0) :
    ∫⁻ x, f x ∂μ = f a * μ {a} := by sorry

end MeasureTheory

Source

Standard measure theory; adapted from material vendored in `Salt/Entropy/` of the Salt project, https://github.com/jyh/salt (Apache-2.0, Jason Hickey), itself derived from the Polynomial Freiman–Ruzsa project (Terence Tao and contributors, Apache-2.0).

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