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Answers for “单调收敛定理是什么?”

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The Monotone Convergence Theorem states that if a sequence (an)n∈N(a_n)_{n\in\mathbb{N}} is monotonically decreasing (i.e., an+1≤ana_{n+1} \le a_n for all nn) and bounded from below, then the sequence is convergent. This is a direct application of the existence of the infimum for bounded sets.

Conditions: The sequence (an)(a_n) is monotonically decreasing.; The sequence (an)(a_n) is bounded from below.

Know when to use it

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The Monotone Convergence Theorem is used after establishing that the sequence is both bounded below and non-increasing. It is invoked to guarantee the existence of a real limit LL for the sequence, which is a prerequisite for solving the limit equation.

Conditions: The sequence has been proven to be bounded from below.; The sequence has been proven to be non-increasing.

Understand why

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The Monotone Convergence Theorem states that every bounded, monotone sequence of real numbers converges. Therefore, to prove convergence using this theorem, one must establish both boundedness and monotonicity as the necessary hypotheses.

Conditions: The sequence consists of real numbers.; The goal is to prove convergence using the Monotone Convergence Theorem.

Understand why

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The bisection method constructs a sequence of upper bounds (bn)(b_n) that is proven to be a Cauchy sequence. However, being Cauchy only means the terms get close to each other; it does not guarantee they converge to a specific real number.

Conditions: The sequence (bn)(b_n) has been proven to be a Cauchy sequence.; The context is the field of real numbers R\mathbb{R}.

Meet the concept

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The example proves the sequence is non-increasing. The mathematical derivation shows an+1−an≤0a_{n+1}-a_n \le 0, which implies an+1≤ana_{n+1} \le a_n.

Conditions: The sequence is defined by an+1=13(an+1)a_{n+1}=\frac{1}{3}(a_n+1) with a1=1a_1=1.; The lower bound an≥12a_n \ge \frac{1}{2} has been proven.

Understand why

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The inequality −23an+13≤0-\frac{2}{3}a_n+\frac{1}{3}\le0 is algebraically equivalent to 13≤23an\frac{1}{3}\le\frac{2}{3}a_n, which simplifies to 12≤an\frac{1}{2}\le a_n. Since the lower bound an≥12a_n\ge\frac{1}{2} was already proven by induction, the difference an+1−ana_{n+1}-a_n is guaranteed to be non-positive.

Conditions: The expression for the difference is an+1−an=−23an+13a_{n+1}-a_n = -\frac{2}{3}a_n + \frac{1}{3}.; The lower bound an≥12a_n \ge \frac{1}{2} has been established for all n≥1n \ge 1.

Find a method

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The distance between any two terms bnb_n and bmb_m (with m>nm > n) is bounded by the length of the interval [an,bn][a_n, b_n]. Because the bisection method halves the interval length at each step, this distance is bounded by (12)n−1∣b1−a1∣\left(\frac{1}{2}\right)^{n-1}|b_1 - a_1|.

Conditions: m>n∈Nm > n \in \mathbb{N}.; The sequences (an)(a_n) and (bn)(b_n) are constructed via the bisection method.; bnb_n is an upper bound and ana_n is a lower bound for the set MM.