Consider the sequence \(a_n=4+\frac{10(-1)^n}{n}\) for positive integers \(n\).
1) Find \(a_1\), \(a_2\), \(a_{10}\), and \(a_{100}\).
2) Find the limit \(g\) as \(n\to\infty\).
3) Find the first index from which every remaining term lies in the interval \((3.95, 4.05)\).
4) Explain why the sequence is bounded but not monotonic.
Hints
- Determine what happens to the fractional term as \(n\) grows.
- Rewrite the interval condition as \(|a_n-g|<\varepsilon\).
- Review the definition of a monotonic sequence.
- To show boundedness, find fixed lower and upper bounds for all terms.
Solution
1. Substitution gives
\(a_1=-6\), \(a_2=9\), \(a_{10}=5\), and \(a_{100}=4.1\).
2. Since \(\frac{10(-1)^n}{n}\to0\), the sequence converges to \(g=4\).
3. The interval condition is equivalent to \(|a_n-4|<0.05\). Thus,
\(\left|\frac{10(-1)^n}{n}\right|=\frac{10}{n}<0.05\).
This requires \(n>200\), so every term beginning with \(a_{201}\) lies in the interval.
4. The odd-indexed terms lie below \(4\), and the even-indexed terms lie above \(4\), so the sequence is not monotonic. Also, the largest deviation occurs among the first terms: \(-6\le a_n\le9\). Therefore, it is bounded.
Answer
1) \(a_1=-6\), \(a_2=9\), \(a_{10}=5\), and \(a_{100}=4.1\).
2) \(g=4\).
3) Every remaining term lies in the interval beginning at \(n=201\).
4) The sequence is bounded by \(-6\) and \(9\), but it alternates around its limit and is not monotonic.