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Assume that \(\lim _{n \rightarrow \infty} a_{n}\) exists. Find all fixed points of \(\left\\{a_{n}\right\\}\), and use a table or other reasoning to guess which fixed point is the limiting value for the given initial condition. $$ a_{n+1}=2 a_{n}\left(1-a_{n}\right), a_{0}=0.1 $$

Short Answer

Expert verified
The limiting value of the sequence is \(\frac{1}{2}\) for \(a_0 = 0.1\).

Step by step solution

01

Understanding Fixed Points

A fixed point of a sequence is a value \(a\) such that when substituted into the recurrence relation, it results in the same value. For the given relation \(a_{n+1} = 2a_n(1-a_n)\), we set \(a = 2a(1-a)\) to find the fixed points.
02

Solving the Fixed Point Equation

Simplify the fixed point equation: \[ a = 2a(1-a) \] leads to \[ a = 2a - 2a^2 \]. Rearrange to form \[ 2a^2 - a = 0 \]. Factoring gives us \[ a(2a-1) = 0 \]. Thus, the fixed points are \(a = 0\) and \(a = \frac{1}{2}\).
03

Analyzing Stability of Fixed Points

To determine stability, calculate the derivative \( f'(a) = 2 - 4a \). Evaluate at fixed points: \( f'(0) = 2 \) (unstable since \( |f'(0)| > 1 \)) and \( f'(\frac{1}{2}) = 0 \) (stable since \( |f'(\frac{1}{2})| < 1 \)).
04

Iterating to Find Limiting Value

Start with \(a_0 = 0.1\) and iterate: Compute \(a_1 = 2(0.1)(0.9) = 0.18\), \(a_2 = 2(0.18)(0.82)\), continuing until the values stabilize. This suggests the sequence approaches the stable fixed point \(a = \frac{1}{2}\).
05

Final Conclusion

Given the iterations stabilize, the limiting value for \(a_0 = 0.1\) is the stable fixed point \(\frac{1}{2}\). Thus, for the initial condition \(a_0 = 0.1\), the sequence \(a_n\) converges to \(\frac{1}{2}\).

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Key Concepts

These are the key concepts you need to understand to accurately answer the question.

Sequence Convergence
Convergence in sequences refers to the behavior of a sequence as its terms approach a particular value, known as the limit, as the sequence progresses towards infinity. In the context of the given problem, we start with an initial value, and through continuous application of the recurrence relation, the sequence generates successive terms. The convergence of this sequence is indicated by the sequence stabilizing or consistently approaching a specific value.

In mathematical terms, a sequence \(a_n\) is said to converge to a limit \(L\) if for every positive number \(\varepsilon\), there exists an integer \(N\) such that for all \(n \geq N\), the terms of the sequence satisfy \(|a_n - L| < \varepsilon\). In simpler terms, as the sequence progresses, the distance between the sequence terms and the limiting value becomes arbitrarily small.
  • For the given sequence \(a_{n+1}=2a_n(1-a_n)\) with initial value \(a_0 = 0.1\), the terms converge towards the limit as demonstrated by iteration, which is the stable fixed point found through analysis.
Stability Analysis
Stability analysis involves examining whether a fixed point attracts or repels nearby points. For a fixed point to be stable, small deviations from this point should return to it in successive iterations. We do this by analyzing the derivative of the function associated with the recurrence relation.

For the sequence defined by \(a_{n+1} = 2a_n(1-a_n)\), we calculate the derivative \(f'(a) = 2 - 4a\). By evaluating this derivative at the fixed points, we assess their stability:
  • At \(a=0\), \(f'(0) = 2\), which is greater than 1 in absolute value, indicating that this fixed point is unstable. Essentially, any small perturbation away from \(0\) leads the sequence away from this point.
  • At \(a=\frac{1}{2}\), \(f'(\frac{1}{2}) = 0\), which means this fixed point is stable as deviations from it tend to shrink back, converging to the fixed point.
This stability analysis helps predict the behavior of the sequence near fixed points and suggests the limiting value.
Recurrence Relation
A recurrence relation provides a way to generate a sequence by repeatedly applying a fixed rule or formula. It describes each term in terms of the previous ones. For our problem, the recurrence relation is given by \(a_{n+1} = 2a_n(1-a_n)\). This means that each subsequent term is produced by substituting the value of the current term into this equation.

Recurrence relations are a powerful tool in mathematics because they systematically define infinite sequences without needing explicit equations for each term. By starting with an initial condition, such as \(a_0 = 0.1\), the entire sequence can be produced by recursively applying the relation.
  • The recurrence formula in this case models a logistic map, a classic example used to study chaotic systems and biological growth models, among other phenomena.
  • The key to understanding them lies in observing patterns and fixed points, as well as examining their stability over iterations.
Through such relations, we can delve into complex behavior of sequences arising from simple iterative processes.
Limiting Value
The limiting value of a sequence is essentially the value that the terms of the sequence get closer to and eventually stabilize at, as the sequence progresses. Identifying limiting values is crucial for understanding the long-term behavior of sequences.

In the problem at hand, the limiting value is derived from the behavior of the sequence as it iterates from the initial condition. The fixed points that are stable act as potential candidates for limiting values. After testing stability, only the stable fixed points concern us as potential limits.
  • Starting from \(a_0 = 0.1\), iterations lead to sequential values approaching \(\frac{1}{2}\). This is evidenced by repeated application of the recurrence formula which stabilizes at \(\frac{1}{2}\).
  • Therefore, we consider \(\frac{1}{2}\) as the limiting value, as it is the stable fixed point that the sequence converges to over time.
The identification of limiting values helps predict how sequences behave in the long run, making them an essential part of sequence analysis.

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Most popular questions from this chapter

Assume that the population growth is described by the Beverton-Holt recruitment curve with parameters \(R_{0}\) and a. Find the population sizes for \(t=1,2, \ldots, 5\) and find \(\lim _{t \rightarrow \infty} N_{t}\) for the given initial value \(N_{0} .\) \(R_{0}=4, a=1 / 60, N_{0}=2\)

Use the limit laws to determine \(\lim _{n \rightarrow \infty} a_{n}=a .\) $$ \lim _{m \rightarrow \infty}\left(\frac{2}{n}-\frac{3}{n^{2}+1}\right) $$

Mountain Gorilla Conservation You are trying to build a mathematical model for the size of the population of mountain gorillas in a national park in Uganda. The data in this question are taken from Robbins et al. (2009). (a) You start by writing a word equation relating the population of gorillas \(t\) years after the study begins, \(N_{t}\), to the population \(N_{t+1}\) in the next year: \(N_{t+1}=N_{t}+\begin{array}{l}\text { number of gorillas } \\ \text { born in one year }\end{array}-\begin{array}{l}\text { number of gorillas } \\\ \text { that die in one yeai }\end{array}\) We will derive together formulas for the number of births and the number of deaths. (i) Around half of gorillas are female, \(75 \%\) of females are of reproductive age, and in a given year \(22 \%\) of the females of reproductive age will give birth. Explain why the number of births is equal to: \(\quad 0.5 \cdot 0.75 \cdot 0.22 \cdot N_{t}=0.0825 \cdot N_{t}\) (ii) In a given year \(4.5 \%\) of gorillas will die. Write down a formula for the number of deaths. (iii) Write down a recurrence equation for the number of gorillas in the national park. Assuming that there are 300 gorillas initially (that is \(N_{0}=300\) ), derive an explicit formula for the number of gorillas after \(t\) years. (iv) Calculate the population size after \(1,2,5\), and 10 years. (v) According to the model, how long will it take for population size to double to 600 gorillas? (b) In reality the population size is almost totally stagnant (i.e., \(N_{t}\) changes very little from year to year). Robbins et al. (2009) consider three different explanations for this effect: (i) Increased mortality: Gorillas are dying sooner than was thought. What percentage of gorillas would have to die each year for the population size to not change from year to year? (ii) Decreased female fecundity: Gorillas are having fewer offspring than was thought. Calculate the female birth rate (percentage of reproductive age females that give birth) that would lead to the population size not changing from year to year. Assume that all other values used in part (a) are correct. (iii) Emigration: Gorillas are leaving the national park. What number of gorillas would have to leave the national park each year for the population to not change from year to year?

Write each sum in sigma notation. \(\frac{3}{5}+\frac{4}{6}+\frac{5}{7}+\frac{6}{8}+\frac{7}{9}\)

Assume that the population growth is described by the Beverton-Holt recruitment curve with parameters \(R_{0}\) and a. Find the population sizes for \(t=1,2, \ldots, 5\) and find \(\lim _{t \rightarrow \infty} N_{t}\) for the given initial value \(N_{0} .\) \(R_{0}=4, a=1 / 40, N_{0}=2\)

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