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Determine whether the statement is true or false. Explain your answer. A sequence \(\left\\{a_{n}\right\\}\) is monotone if \(a_{n+1}-a_{n} \neq 0\) for all \(n \geq 1\).

Short Answer

Expert verified
The statement is false; the condition does not guarantee monotonicity.

Step by step solution

01

Definition of a Monotone Sequence

A sequence \( \{a_n\} \) is called monotone if it is either entirely non-increasing or non-decreasing. This means \( a_{n+1} \geq a_n \) for all \( n \) if the sequence is non-decreasing, or \( a_{n+1} \leq a_n \) for all \( n \) if the sequence is non-increasing.
02

Analyze the Given Condition

The condition given is \( a_{n+1} - a_n eq 0 \). This implies that for each \( n \), \( a_{n+1} \) is not equal to \( a_n \). However, this condition does not specify whether \( a_{n+1} \) should be strictly greater than or strictly less than \( a_n \), only that they are not equal.
03

Relation between Condition and Monotonicity

A sequence having \( a_{n+1} - a_n eq 0 \) does not ensure monotonicity. For example, alternating numbers like 1, 2, 1, 2,... satisfy \( a_{n+1} - a_n eq 0 \) but aren't monotone. Monotonicity requires a consistent direction (non-increasing or non-decreasing), not just inequality.
04

Conclusion about the Given Statement

The statement is false. The condition \( a_{n+1} - a_n eq 0 \) by itself does not ensure that the sequence is monotone because it does not restrict the changes to be in one consistent direction.

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Key Concepts

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

Increasing sequence
An increasing sequence is a type of monotone sequence where each term is equal to or greater than the previous term. In mathematical terms, a sequence \( \{a_n\} \) is considered non-decreasing if for every index \( n \), the inequality \( a_{n+1} \geq a_n \) holds true.
When a sequence is strictly increasing, it means that each term is greater than the previous one, denoted as \( a_{n+1} > a_n \).
  • Example: The sequence 2, 4, 6, 8,... is strictly increasing since each term increases by 2.
To determine if a sequence is increasing, one must check this inequality condition for all terms in the sequence. This consistent upward trend ensures the sequence is non-decreasing or monotonic in an increasing sense.
Decreasing sequence
Conversely, a decreasing sequence is another type of monotonic sequence where each subsequent term is less than or equal to the previous term. Formally, a sequence \( \{a_n\} \) is non-increasing if for all \( n \), the inequality \( a_{n+1} \leq a_n \) holds true.
In the case of a strictly decreasing sequence, every term is less than the term before it, satisfying \( a_{n+1} < a_n \).
  • Example: The sequence 5, 3, 1, -1,... is strictly decreasing since each term decreases by 2.
For a sequence to be deemed decreasing, it must consistently follow this non-increasing pattern throughout all of its terms, ensuring a steady downward trajectory.
Mathematical proof
Mathematical proof is a logical argument that establishes the truth of a given statement. It involves systematically demonstrating that certain conditions or assertions lead to a conclusion.
In the context of sequences, to prove whether a sequence is monotone, one can check whether it fulfills the criteria for being non-decreasing or non-increasing, as discussed in the respective sections on increasing and decreasing sequences.
A proof typically involves:
  • Assumptions: Identifying given conditions or facts that are considered true.
  • Logical Sequence: Using established rules and logical reasoning to connect the facts.
  • Conclusion: Arriving at a statement that definitively establishes whether the sequence is monotonic.
For instance, in examining the sequence condition \( a_{n+1} - a_n eq 0 \), the proof shows it doesn’t ensure monotonicity because it doesn’t maintain a consistent increase or decrease for all terms.

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