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Equipartition identity with explicit integrated boundary terms
Statement
Let be a sigma-finite measure space, , and a product-measurable function whose map is for -almost every . Set where is Lebesgue measure on times . Assume For let and assume as . Define whenever the integral is absolutely convergent. Then For finite endpoints , assume the endpoint integral below is finite and put . If , define . Then the exact boundary identity is
Facts & Assumptions
Given: The product measure space, , , the finite positive normalization , the integrable moment, and the vanishing integrated boundary term .
The measure factors are sigma-finite, is measurable and sectionwise , and and are finite with .
The two integrated endpoint contributions tend to zero, equivalently .
For the finite-interval clause, and the stated integrated endpoint expression is finite.
Tonelli's theorem permits section integration for nonnegative product-measurable functions on sigma-finite factors (Tonelli's theorem for nonnegative measurable functions on a sigma-finite product).
Dominated convergence passes limits through an integral under one common integrable majorant (Dominated convergence).
Proof
Given: [A1]–[A3] and the Tonelli and dominated-convergence interfaces [F1]–[F2].
For almost every , differentiate the product in the coordinate: The absolute value of the right side is bounded by , which is integrable by [A1].
Integrate this identity on . Absolute integrability and [F1] justify integrating first in and then in , so
The integrand in the first term is nonnegative and integrable, while the second integrand is absolutely integrable by [A1]. Their restrictions to converge pointwise almost everywhere to the full integrands and are dominated by those integrable functions. Applying [F2] to both terms and using [A2] gives . Divide by to obtain the stated identity.
For finite , integrate the same derivative over . The boundary evaluation is Dividing by and rearranging yields the displayed finite-endpoint formula.