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Mathematics · unit version 1

Weighted integration and jump conventions

Construct Riemann–Stieltjes sums, prove existence under explicit hypotheses, and distinguish refinement convergence from arbitrary fine-mesh convergence.

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Readiness and preparation

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Why does refinement tighten ordinary lower and upper sums?

Weighted sums reuse containment of extrema but replace widths with integrator increments.

Compare reasoning and find a refresher

Each child cell has a higher infimum and lower supremum; its nonnegative weight contributes to a sum whose weights add to the parent weight.

Review Darboux sums and refinement

Does monotonicity prescribe the value at an interior jump?

The point value determines which adjacent cell receives each part of the increment.

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It only places that value between the two one-sided limits. Different allowed values can redistribute the jump weight.

Review one-sided limits and jumps

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Mixed weights, jump values and existence boundaries

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Lesson sequence

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  1. Weighted sums and refinement

    Lesson details and objectives

    Build weighted Darboux integration from nonnegative increments, prove its refinement criterion, and state exactly how arbitrary tagged sums converge.

    • Prove weighted refinement and cross-partition bounds using nonnegative additive increments.
    • Establish the small-gap criterion and its equivalent convergence directed by refinement.
    • Prove continuous-function existence with a modulus estimate and handle zero total weight.

    6 practice tasks.

  2. Jumps, tags and endpoint conventions

    Lesson details and objectives

    Separate refinement integrability from all-small-mesh tagged convergence and account for the exact mass carried by interior and endpoint jumps.

    • Prove the relation between weighted Darboux integrability and tagged sums over refinements, and distinguish the stronger mesh condition.
    • Analyze a jump whose point-value convention permits refinement integrability and a changed integrand that prevents it.
    • Derive interior and endpoint jump contributions for continuous integrands from the actual partition increments.

    6 practice tasks.

  3. Prove existence while keeping the definitions separate

    Lesson details and objectives

    Control weighted gaps and every sufficiently fine tagged sum, including finite discontinuities away from integrator jumps.

    • Prove both weighted Darboux integrability and mesh-based tagged convergence for continuous integrands.
    • Prove the finite-discontinuity existence theorem using small integrator increments and a compact good set.
    • Audit shared jumps, endpoint continuity and the difference between a sufficient condition and a necessary condition.

    6 practice tasks.

  4. Reduce smooth weights and prove integration by parts

    Lesson details and objectives

    Use a derivative density only when justified, define signed smooth integrators, and derive boundary terms from sums.

    • Prove the C¹ nondecreasing-integrator reduction from weighted tagged sums.
    • Construct the signed bounded-variation integral for continuous integrands and prove independence of its monotone decomposition.
    • Derive integration by parts for continuous bounded-variation functions and explain why jumps are not recovered from an almost-everywhere derivative.

    6 practice tasks.