A bounded monotone tail and the supremum argument

On this page
  1. Goal and scope
  2. Givens and symbols
  3. Rules and sources
  4. Main derivation
  5. Substeps and lemmas
  6. Reverse audits
  7. Error controls and negative controls
  8. Result and limitations
  9. Verification and source binding

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derivation-mko-monotone-sequence-convergence-theorem-01 · ocme-new142-reader-adoption-20261021

Goal and scope

For a_n=3-2/(n+1), prove monotonicity, boundedness, supremum 3 and convergence.

A full n≥0 law with a strict epsilon certificate; explicitly include the general theorem's supremum step.

Givens and symbols

  • algebra
  • epsilon
  • supremum
  • definition_here
  • G1 — a_n=3-2/(n+1) for every n≥0; use the least-upper-bound property of R.
  • a_n — concrete whole sequence
  • s — general theorem supremum

Rules and sources

Some rule statements have reader clarifications in this revision: they are paraphrases, not verbatim quotations. Referenced original source bytes and hashes are unchanged.

  • mko_definition · cited — Keep whole-sequence monotonicity and the corresponding bound. If s is the supremum of the terms, s−epsilon is not an upper bound, supplying an index N; monotonicity then controls every later term. A supremum or infimum need not be a term. A finite prefix does not supply a universal certificate. A monotone sequence without the required bound, or a bounded sequence lacking monotonicity, does not meet this theorem's hypotheses. · mko-monotone-sequence-convergence-theorem
  • algebra · prerequisite — Use real-field identities; division requires a nonzero denominator and multiplication by a positive number preserves inequalities.
  • epsilon · prerequisite — Convergence requires for every positive epsilon an integer N controlling every n≥N by a strict error bound; use the Archimedean property of the reals.
  • supremum · prerequisite — A nonempty bounded-above real set has supremum s; for epsilon>0, s-epsilon is not an upper bound.
  • definition_here · prerequisite — The epsilon definition of real-sequence limit L requires, for each positive epsilon, an N with |u_n-L|<epsilon for every n≥N.

Main derivation

S01 · implies

Before

a_{n+1}-a_n=2/(n+1)-2/(n+2)

After

a_{n+1}-a_n=2/((n+1)(n+2))>0

Conditions: n\ge0

Rule: algebra · Uses: G1

Leaf basis: algebra

Both denominators are positive, proving every adjacent difference positive and hence whole-sequence strict increase.

S02 · implies

Before

0<2/(n+1)\le2

After

1\le a_n<3

Conditions: none

Rule: algebra · Uses: G1

Leaf basis: algebra

This gives lower bound 1 and the upper bound 3 needed for the increasing-sequence theorem.

S03 · implies

Before

\varepsilon>0,\quad N=\lfloor2/\varepsilon\rfloor+1

After

0<3-a_n=2/(n+1)<\varepsilon\quad(n\ge N)

Conditions: none

Rule: epsilon · Uses: G1, S02

Leaf basis: epsilon

Every later denominator is larger, directly proving a_n tends to 3.

S04 · implies

Before

u<3,\quad\varepsilon=3-u>0

After

a_N>3-\varepsilon=u;\quad\sup\{a_n:n\ge0\}=3

Conditions: none

Rule: supremum · Uses: S02, S03

Leaf basis: supremum

Every number below 3 fails as an upper bound; combined with the upper-bound check this gives leastness.

S05 · implies

Before

(u_n)\text{ nondecreasing and bounded above},\quad s=\sup\{u_n\}

After

\forall\varepsilon>0\ \exists N:\ s-\varepsilon<u_N\le s

Conditions: (u_n)\text{ nondecreasing and bounded above}

Rule: supremum · Uses: G1

Leaf basis: supremum

For the general theorem's core, s-epsilon is not an upper bound, so some term exceeds it.

S06 · implies

Before

n\ge N

After

s-\varepsilon<u_N\le u_n\le s

Conditions: none

Rule: algebra · Uses: S05

Leaf basis: algebra

Whole-sequence monotonicity transfers one close term to the entire tail.

S07 · definition

Before

0\le s-u_n<\varepsilon\quad(n\ge N)

After

u_n\to s

Conditions: none

Rule: definition_here · Uses: S06

Leaf basis: definition_here

The absolute error equals s-u_n, meeting the limit definition and completing the supremum proof.

S08 · implies

Before

a_n\text{ nondecreasing},\quad\sup\{a_n\}=3

After

\lim a_n=3\text{, not attained by any }a_n

Conditions: none

Rule: epsilon · Uses: S01, S02, S04, S07

Leaf basis: epsilon

The general proof and direct tail agree on the limit, retaining the nonattainment boundary.

Substeps and lemmas

No substeps in this guide.

Reverse audits

  • S08 · premise_audit · S01, S02, S04, S07 — No converse claiming all convergent sequences monotone is inferred.
  • S07 · premise_audit · S06 — Only the stated monotonicity and bound premises are used, not inferred from samples.
  • S06 · premise_audit · S05 — Without monotonicity, u_N cannot control all later terms.
  • S05 · premise_audit · G1 — This separately names a general u_n and both premises, avoiding symbol collision with a_n.
  • S04 · premise_audit · S02, S03 — A supremum need not be a term; no term equals 3 here.
  • S03 · premise_audit · G1, S02 — N is fixed once for the entire later tail.
  • S02 · premise_audit · G1 — The upper bound is not a maximum because every term is strictly below 3.
  • S01 · premise_audit · G1 — The formula holds for every n, not sampled neighbors.

Error controls and negative controls

Paired premises

Pair nondecreasing with an upper bound and nonincreasing with a lower bound.

Strict bound

Failure of s-epsilon as an upper bound supplies a strictly close term.

  • a_n has maximum 3. → reject — Every term is strictly below 3.
  • Monotonicity alone gives a real limit. → reject — n increases without an upper bound and has no finite limit.
  • Boundedness alone gives convergence. → reject — (-1)^n is bounded with different parity limits.

Result and limitations

a_n is strictly increasing and bounded, tends to 3, and has unattained supremum 3.

    Verification and source binding

    1 declared checks for this guide; only the listed expressions are covered.

    • source commit: b1b2f1dcf8c50d67a93fe52d95f8eef7b3abba9f
    • adopted source SHA-256: 98d1cd85cc12d0b9cfee281f8c6886435739d36643e5d46da812d63115e96974
    • original source SHA-256: 318f43093b381e02533751c73966ee4bcfc677e16c35ebd8ca54ffcfe2049934
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