2. Let {sn}=1 be an unbounded sequence of negative numbers. Show {sn}n=1 has a 00 00 subsequence {Sn, such that {Sng } tends to minus infinity k=1 k=1

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Chapter2: Second-order Linear Odes
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# Mathematical Sequence Problems

Below are a series of mathematical problems related to sequences, designed for educational purposes. These problems explore concepts like convergent sequences, Cauchy sequences, and the usual metric in \( \mathbb{R} \).

### Problem Set

1. **Convergent Sequence Using the Definition:**
   Use the definition of a convergent sequence to show the following sequence converges to 7:
   \[
   \left\{ 7 - \frac{4}{n} \right\}_{n=1}^\infty
   \]
   This is considered to be a sequence in \( \mathbb{R} \) with the usual metric \( d_{\mathbb{R}}(x, y) = |x - y| \).

2. **Subsequence Tending to Minus Infinity:**
   Let \( \{s_n\}_{n=1}^\infty \) be an unbounded sequence of negative numbers. Show \( \{s_n\}_{n=1}^\infty \) has a subsequence \( \{s_{n_k}\}_{k=1}^\infty \) such that \( \{s_{n_k}\}_{k=1}^\infty \) tends to minus infinity.

3. **Cauchy Sequences in \( \mathbb{R} \) and \( \mathbb{R}^4 \):**
   Let \( \{a_n\}_{n=1}^\infty \), \( \{b_n\}_{n=1}^\infty \), \( \{c_n\}_{n=1}^\infty \), and \( \{d_n\}_{n=1}^\infty \) be Cauchy sequences in \( \mathbb{R} \) with the usual metric. Show \( \{x_n\}_{n=1}^\infty \) where \( x_n = (a_n, b_n, c_n, d_n) \) is Cauchy in \( \mathbb{R}^4 \).

4. **Cauchy Sequence Proof:**
   Prove the following sequence is a Cauchy sequence in \( \mathbb{R} \) with the usual metric \( d_{\mathbb{R}}(x, y) = |x - y| \):
   \[
   \left\{
Transcribed Image Text:# Mathematical Sequence Problems Below are a series of mathematical problems related to sequences, designed for educational purposes. These problems explore concepts like convergent sequences, Cauchy sequences, and the usual metric in \( \mathbb{R} \). ### Problem Set 1. **Convergent Sequence Using the Definition:** Use the definition of a convergent sequence to show the following sequence converges to 7: \[ \left\{ 7 - \frac{4}{n} \right\}_{n=1}^\infty \] This is considered to be a sequence in \( \mathbb{R} \) with the usual metric \( d_{\mathbb{R}}(x, y) = |x - y| \). 2. **Subsequence Tending to Minus Infinity:** Let \( \{s_n\}_{n=1}^\infty \) be an unbounded sequence of negative numbers. Show \( \{s_n\}_{n=1}^\infty \) has a subsequence \( \{s_{n_k}\}_{k=1}^\infty \) such that \( \{s_{n_k}\}_{k=1}^\infty \) tends to minus infinity. 3. **Cauchy Sequences in \( \mathbb{R} \) and \( \mathbb{R}^4 \):** Let \( \{a_n\}_{n=1}^\infty \), \( \{b_n\}_{n=1}^\infty \), \( \{c_n\}_{n=1}^\infty \), and \( \{d_n\}_{n=1}^\infty \) be Cauchy sequences in \( \mathbb{R} \) with the usual metric. Show \( \{x_n\}_{n=1}^\infty \) where \( x_n = (a_n, b_n, c_n, d_n) \) is Cauchy in \( \mathbb{R}^4 \). 4. **Cauchy Sequence Proof:** Prove the following sequence is a Cauchy sequence in \( \mathbb{R} \) with the usual metric \( d_{\mathbb{R}}(x, y) = |x - y| \): \[ \left\{
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