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Answers for “在这个递归数列的证明中,哪里使用了单调收敛定理?”

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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.

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.