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Answers for “如何计算递推关系 $a_{n+1}=\frac{1}{3}(a_n+1)$ 的 $a_{n+1}-a_n$?”

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Subtract ana_n from both sides of the recurrence relation. Distribute the 13\frac{1}{3} and combine like terms to simplify the expression to −23an+13-\frac{2}{3}a_n + \frac{1}{3}.

Conditions: The recurrence relation is an+1=13(an+1)a_{n+1}=\frac{1}{3}(a_n+1).; The goal is to analyze the sign of the difference to determine monotonicity.

Find a method

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To compute the terms, substitute the initial value into the recurrence relation iteratively. First, plug in a1=1a_1=1 to find a2a_2.

Conditions: The recurrence relation is an+1=13(an+1)a_{n+1}=\frac{1}{3}(a_n+1).; The initial value is a1=1a_1=1.; The index nn starts at 11.

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The proof uses mathematical induction. First, the base case is verified by checking that a1=1≥12a_1=1 \ge \frac{1}{2}.

Conditions: The sequence is defined by an+1=13(an+1)a_{n+1}=\frac{1}{3}(a_n+1) for n≥1n \ge 1.; The initial value is a1=1a_1=1.; The goal is to prove an≥12a_n \ge \frac{1}{2} for all n≥1n \ge 1.

Find a method

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Once convergence to a limit LL is established, pass to the limit in the recurrence relation an+1=13(an+1)a_{n+1}=\frac{1}{3}(a_n+1). Since both ana_n and an+1a_{n+1} approach LL, substitute LL for both terms to get L=13(L+1)L=\frac{1}{3}(L+1), and then solve this linear equation for LL.

Conditions: The sequence is known to converge to some real number LL.; The recurrence relation is an+1=13(an+1)a_{n+1}=\frac{1}{3}(a_n+1).

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.

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 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.

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.