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Two integrable functions are equal almost everywhere exactly when all of their indefinite integrals agree
Statement
Let . Then the following are equivalent:
- almost everywhere;
- for every measurable ,
For integrable real or complex , the notation in condition 2 means ; the product is integrable because .
Facts & Assumptions
Given: Integrable functions .
The integral over a null set vanishes for nonnegative integrands (A nonnegative integral over a null set vanishes).
The Lebesgue integral is linear on (The Lebesgue integral is linear on ).
A nonnegative measurable function has integral exactly when it vanishes almost everywhere (A nonnegative measurable function has integral exactly when it vanishes almost everywhere).
Real and imaginary parts of an integrable complex function are integrable (Integrable real and complex functions, and their integrals).
Proof
Assume almost everywhere, with exceptional null set . For every measurable , the positive and negative parts of the real and imaginary components of are supported on . By [L1] their nonnegative integrals vanish, and [L2] and [L4] give , hence .
Assume instead that for every measurable . Apply this to the real part on and to on . In each case the corresponding nonnegative integral is , so [L3] gives almost everywhere. The same argument for shows almost everywhere. Hence almost everywhere.
Step 1.1 proves and step 1.2 proves .
Depends on
Used by
- Dominated convergence is a Vitali corollary Corollary
- One-dimensional W^1,p functions have unique absolutely continuous representatives Corollary
- Polar integration may discard the cut locus Corollary
- Pointwise shock values do not affect the weak solution Counterexample
- BMO seminorm and the quotient by constants Definition
- Conditional expectation as an ae class Definition
- Direct integral of a measurable Hilbert field Definition
- Fourier coefficients and trigonometric polynomials on the torus Definition
- Natural and usual augmented Brownian filtrations Definition
- The heat evolution Hₜ of initial data Definition
- The Lusin area function for a fixed admissible kernel and aperture Definition
- The Poisson integral of a finite complex boundary measure Definition
- Compact groups have a constant Reiter net Example
- Piecewise-affine approximation of a measurable coefficient Example
- Projective-plane curvature via a hemisphere Example
- FALSE: every increasing function satisfies Newton-Leibniz with its derivative False statement
- Centring by translation and modulation preserves the variance product Lemma
- Compact support gives an entire Fourier-Laplace transform by slices Lemma
- Complex translation, convolution, approximate identities, and mollification Lemma
- Conditional variance is well-defined and has the second-moment formula Lemma
- Convolution on L¹(ℝⁿ) is independent of the chosen Borel representatives Lemma
- Expectation depends only on the almost-everywhere class Lemma
- Finite Rademacher blocks are equidistributed Lemma
- Gaussian decay gives an entire Fourier-Laplace transform and its growth bound Lemma
- L² with the integral pairing is a Hilbert space Lemma
- Near and far bounds for a Riesz potential Lemma
- Real L2 multipliers and unitary transport Lemma
- Sobolev functions paste across an overlap Lemma
- The integral transform is representative independent Lemma
- Weak differentiation ignores null-set changes Lemma
- Compact and locally compact abelian groups are amenable Proposition
- Dominated families are uniformly integrable Proposition
- A bounded Riemann integrable function on a closed bounded interval is Lebesgue measurable and has the same integral Theorem
- Complex Holder, Minkowski, and the quotient norm Theorem
- Every g∈ L^q(μ) defines a bounded linear functional on Lᵖ(μ) Theorem
- For a nondecreasing function, the derivative is measurable and integrable and its integral is bounded by the total increase Theorem
- Gibbs overshoot at a piecewise C¹ jump Theorem
- Hardy–Littlewood–Sobolev fractional integration inequality Theorem
- Hᵏ is a Hilbert space under the derivative-sum inner product Theorem
- Kolmogorov convergence criterion Theorem
…and 5 more results.
Dependency tree · two levels
14 results within two dependency steps of this one, each drawn at its shortest distance from it. An arrow runs from a result to what uses it, so the chart reads left to right and ends at this result, which carries a heavier outline. Every node is a link to that result. Click elsewhere on the chart to enlarge it.
Sources
- Richard F. Bass, Real Analysis for Graduate Students, Proposition 8.2 (standard reference, not scraped)
- Gerald B. Folland, Real Analysis, 2nd ed., Proposition 2.23(b) (standard reference, not scraped)