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Mathematics · unit version 1

Tail limits and root tests

Construct upper and lower limits from entire tails, characterize subsequential behavior, and prove convergence tests without assuming ordinary root limits exist.

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Readiness and preparation

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Must a supremum be a value attained by a term?

Constructing endpoint subsequences requires approximate bounds, not an assumed largest term.

Compare reasoning and find a refresher

No. The values 1−1/n have supremum 1 without ever attaining it. A supremum supplies arbitrarily close values from below when finite.

Review supremum and infimum

What must be checked before using the ordinary-limit root test?

Sparse or alternating roots may have no ordinary limit.

Compare reasoning and find a refresher

The ordinary root limit must exist. This unit replaces that premise with an upper limit and proves the resulting convergence and divergence cases.

Review ratio and ordinary root tests

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Construct the cluster set and audit a sparse power series

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Lesson sequence

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  1. Tail extrema and subsequential limits

    Lesson details and objectives

    Define limsup and liminf using infinite tails, construct their finite or infinite subsequential limits, and separate sampled bounds from eventual claims.

    • Define monotone extended-real tail extrema and prove eventual and infinitely-often characterizations.
    • Construct increasing-index subsequences approaching both endpoints without assuming suprema or infima are attained.
    • Characterize convergence through equal endpoints and distinguish endpoint data from a complete subsequential-limit set.

    6 practice tasks.

  2. Test series using upper limits of roots

    Lesson details and objectives

    Turn an eventual root bound into an absolute tail estimate, detect infinitely recurring large terms, and keep the boundary and sparse-index cases honest.

    • Prove the extended-limsup root test using eventual and infinitely-often inequalities.
    • Analyze sparse and oscillating root sequences without assuming an ordinary root or ratio limit.
    • Derive a power-series radius from coefficient roots while testing boundary points separately.

    6 practice tasks.