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Mathematics · unit version 1

Collisions, collection and waiting times

Build probability estimates from the sampling mechanism, distinguish events from counts, and turn average waiting times into carefully justified deadline bounds.

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Readiness and preparation

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Does adding expectations require independent quantities?

Collision indicators and collection stages need a justified sampling mechanism but expectation addition has broader validity.

Compare reasoning and find a refresher

No. Linearity holds for finite sums with finite expectations. Independence is a separate hypothesis used to derive the probabilities of the component events or waiting times.

Review discrete distributions and geometric waiting times

How quickly do harmonic sums grow?

Collection time connects waiting stages to the reciprocal sum.

Compare reasoning and find a refresher

For positive integers N, ln(N+1) ≤ H_N ≤ 1+ln N. These bounds explain logarithmic growth without making a finite mean a guaranteed deadline.

Review harmonic sums and bounds

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Audit the sampling mechanism and its guarantees

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Lesson sequence

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  1. Collision products and birthday bounds

    Lesson details and objectives

    Count repeated labels exactly, distinguish collision counts from collision events, and certify a controlled exponential approximation.

    • Derive the no-collision product from an explicitly independent uniform sampling model, including boundary cases.
    • Compare an indicator expectation, a union bound and an exponential probability bound without equating their meanings.
    • Control logarithmic approximation error and diagnose changes caused by nonuniform or dependent labels.

    6 practice tasks.

  2. Collect coupons by decomposing waiting stages

    Lesson details and objectives

    Derive expected collection time and deadline bounds while tracking the assumptions behind uniform sampling.

    • Derive geometric stage means from a tail-sum argument and obtain NH_N by linearity of expectation.
    • Calculate a small collection-time distribution and prove a missing-label union bound for deadlines.
    • Distinguish averages from guarantees and repair the argument for one-label, nonuniform and impossible-label cases.

    6 practice tasks.