Chapter 9: Problem 65
The sequence defined recursively by \(x_{k+1}=x_{k} /\left(1+x_{k}\right)\) occurs in genetics in the study of the elimination of a deficient gene from a population. Show that the sequence whose \(n\) th term is \(1 / x_{n}\) is arithmetic.
Short Answer
Expert verified
The sequence \(y_n = \frac{1}{x_n}\) is arithmetic with a common difference of 1.
Step by step solution
01
Understand the Problem
We need to demonstrate that the sequence \(y_n\), where \(y_n = \frac{1}{x_n}\), forms an arithmetic sequence. An arithmetic sequence has a constant difference between consecutive terms.
02
Express the Recursive Relation
Given the recursive formula \(x_{k+1} = \frac{x_k}{1 + x_k}\), we need to express \(y_n = \frac{1}{x_n}\) using this relationship.
03
Rewrite the Sequence Formula
From the given recursive formula \(x_{k+1} = \frac{x_k}{1 + x_k}\), invert both sides to find \(y_{k+1}\). So, \(y_{k+1} = \frac{1}{x_{k+1}} = \frac{1 + x_k}{x_k}\). This simplifies to \(y_{k+1} = 1 + \frac{1}{x_k} = 1 + y_k\).
04
Verify the Sequence is Arithmetic
From the equation \(y_{k+1} = y_k + 1\), we can see the common difference \(d = y_{k+1} - y_k = 1\). Thus, the sequence is arithmetic because there is a constant difference between each term.
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Key Concepts
These are the key concepts you need to understand to accurately answer the question.
Recursive Sequences
A **recursive sequence** is a series of numbers where each term is defined as a function of the preceding terms. It can be a powerful tool in mathematics for understanding patterns and behaviors in sequences and appears frequently in real-world applications such as population modeling, algorithm design, and more. Understanding a recursive sequence starts with its base case, which usually gives the initial term. From there, each subsequent term is calculated using the recursive formula.
- For the sequence described in the exercise, the recursive formula is given as \(x_{k+1} = \frac{x_k}{1 + x_k}\).
- This shows that each term is derived from the previous term, \(x_k\), using a specific operation that involves both division and addition.
Sequence in Genetics
Genetics often involves analyzing sequences, where the focus might be on DNA or genetic expressions over time. A **sequence in genetics** could involve an iterative process, just like recursive sequences, to study phenomena such as gene expression, elimination, or propagation in generations.In the given exercise, the recursive sequence \(x_{k+1} = \frac{x_k}{1 + x_k}\) is inspired by genetic studies focusing on eliminating a deficient gene in a population. The formula describes how genetic traits might reduce in frequency across generations, making it a useful model in genetics.
- This kind of mathematical model helps biologists understand how quickly or slowly a trait might be reduced in a population.
- By showing that the transformed sequence is arithmetic, we see a linear interaction at play, which simplifies predictions and analysis.
Problem-Solving Steps
Problem-solving in mathematics often follows structured steps. In the context of demonstrating that a sequence is arithmetic, there's a typical pathway to follow.First, it's imperative to **understand the problem**. Recognize the given recursive sequence and the target, which is to prove a new form of the sequence (in this case, \(y_n = \frac{1}{x_n}\)) follows an arithmetic pattern. This allows you to strategize your solution.The next step, **expressing the recursive relation** in a new form, was significantly achieved by inverting the provided formula. This transformed it into a more recognizable equation showing that each term in the new sequence can be obtained by adding a constant (\(1\) in this scenario) to the previous term, thus satisfying the condition for an arithmetic sequence.
- **Verification**, as shown in this exercise, is crucial: once we manipulate the sequences, checking for an arithmetic property (constant difference) is essential.
- Mentally separating each operation and understanding the math behind it helps verify each transformation accurately.