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Mathematics · unit version 1

Discontinuities and Riemann integration

Prove how one-sided limits, oscillation and small interval covers determine integrability, then test the theorem on dense and uncountable exceptional sets.

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Readiness and preparation

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What makes a bounded function Darboux integrable?

The discontinuity criterion will construct partitions with small upper-minus-lower gaps.

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For every positive epsilon there must be a partition whose upper sum minus lower sum is below epsilon.

Review partitions and the Darboux criterion

Must a supremum be attained?

One-sided limits and local oscillation use suprema of entire sets of values.

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No. Approximation to the least upper bound supplies the estimates even if no point attains it.

Review suprema and infima

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Dense jumps, a Cantor set and cancellation

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Lesson sequence

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  1. Monotone limits and countable jumps

    Lesson details and objectives

    Prove one-sided limits and countability of discontinuities from order, with precise endpoint conventions.

    • Construct finite one-sided limits from approximate suprema and infima.
    • Prove jump countability by disjoint value intervals and a finite variation budget.
    • Analyze accumulating jumps and distinguish endpoint continuity from interior two-sided continuity.

    6 practice tasks.

  2. Point oscillation and small interval covers

    Lesson details and objectives

    Measure local variation by all nearby values, locate closed discontinuity thresholds, and budget countable interval covers.

    • Prove continuity is equivalent to zero point oscillation using relative neighborhoods.
    • Prove positive oscillation threshold sets are compact and express the discontinuity set as their countable union.
    • Construct countable null covers with explicit length budgets and prove a nondegenerate interval is not null.

    6 practice tasks.

  3. Prove the discontinuity criterion for Riemann integration

    Lesson details and objectives

    Connect compact oscillation thresholds to short interval covers and small Darboux gaps, proving both directions of the bounded-interval criterion.

    • Prove that Riemann integrability makes every positive oscillation threshold set null, handling partition endpoints separately.
    • Use compact threshold sets, finite open covers and a fine partition to prove the converse.
    • Apply the criterion to endpoint changes, closed exceptional sets and persistent positive oscillation without confusing nullity with cardinality.

    6 practice tasks.

  4. Dense exceptions and integral values

    Lesson details and objectives

    Construct an integrable function with dense discontinuities and identify the exact limits of null-set modification arguments.

    • Prove Thomae’s continuity set using finite bounded-denominator exceptions.
    • Construct small Darboux upper sums despite a dense set of discontinuities.
    • Prove equality of integrals for two integrable functions equal off a null set, and refute the missing-hypothesis version.

    6 practice tasks.