# Introduction to sequences

0/2
##### Intros
###### Lessons
1. Overview:
2. Notation of Sequences
3. Definitions and theorems of Sequences
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##### Examples
###### Lessons
1. Finding the terms of a sequence

Find the first five terms of the following sequences.
1. $a_n=3(-1)^n$
2. $a_n$= $\frac{n+1}{\sqrt{n+1}}$
3. {$cos(\frac{n\pi}{2})$}
2. Finding the formula for a sequence

Find the formula for the general term $a_n$ for the following sequences
1. {$\frac{1}{2}, \frac{1}{3}, \frac{1}{4}, \frac{1}{5}, ...$}
2. {$\frac{1}{2}, \frac{2}{5}, \frac{3}{8}, \frac{4}{11}, ...$}
3. {-1, 4, -9, 16, ... }
3. Convergence and divergence of sequences

Evaluate the limits and determine if the following limits are converging or diverging.
1. $\lim$n →$\infty$ $\frac{(-1)^n}{n^2}$
2. $\lim$n →$\infty$ $6(\frac{1}{2})^n$
3. $\lim$n →$\infty$ $\frac{n^3+n+1}{n^2+1}$
###### Topic Notes
In this lesson, we will talk about what sequences are and how to formally write them. Then we will learn how to write the terms out of the sequences when given the general term. We will also learn how to write the general term when given a sequence. After learning the notations of sequences, we will take a look at the limits of sequences. Then we will take a look at some definitions and properties which will help us take the limits of complicating sequences. These theorems include the squeeze theorem, absolute value sequences, and geometric sequences.

## Introduction to Sequences

Welcome to our exploration of sequences, a fundamental concept in mathematics. Our journey begins with an engaging introduction video that sets the stage for understanding these fascinating mathematical patterns. Sequences are ordered lists of numbers that follow specific rules, and they play a crucial role in various mathematical applications. In this lesson, we'll delve into the notation of sequences, providing you with the tools to read and interpret them effectively. We'll also cover the formal definition of sequences, ensuring a solid foundation for future studies. You'll learn how to write individual terms and develop general formulas for sequences, skills that are essential for solving complex mathematical problems. Understanding sequences is vital as they form the basis for many advanced mathematical concepts and real-world applications. By mastering sequences, you'll enhance your problem-solving abilities and gain insights into the beautiful patterns that underlie mathematics.

## What is a Sequence?

A sequence, in mathematical terms, is a formally defined list of numbers that follow a specific pattern. This ordered collection of elements is typically represented using the notation a1, a2, ..., an-1, an, where a1 denotes the first term, a2 the second term, and so on, with an representing the nth term or the general term of the sequence. This notation allows mathematicians to describe and analyze patterns within number sets efficiently.

Two common types of sequences are arithmetic and geometric sequences. An arithmetic sequence is characterized by a constant difference between consecutive terms. For example, the sequence 2, 5, 8, 11, 14 is arithmetic, with a common difference of 3. On the other hand, a geometric sequence has a constant ratio between consecutive terms. The sequence 2, 6, 18, 54, 162 is geometric, with each term being three times the previous one.

Understanding sequences is crucial in mathematics as they form the foundation for many advanced concepts. They play a vital role in calculus, particularly in the study of series and limits. In algebra, sequences help in solving recurrence relations and exploring patterns in number theory. Moreover, sequences find practical applications in various fields beyond pure mathematics.

In computer science, sequences are fundamental to algorithm design and analysis. They are used in generating pseudorandom numbers, creating encryption keys, and developing sorting algorithms. In physics, sequences describe natural phenomena like the Fibonacci sequence, which appears in the arrangement of leaves on plants. Economics utilizes sequences in financial modeling, such as compound interest calculations and predicting market trends.

The study of sequences also enhances problem-solving skills and logical thinking. By recognizing patterns and relationships between numbers, students develop a deeper understanding of mathematical structures. This skill is transferable to many areas of life, from scientific research to everyday decision-making processes.

As we delve deeper into the world of sequences, we uncover their beauty in describing both simple and complex patterns in nature and human-made systems. Whether it's the spiral arrangement of sunflower seeds following the Fibonacci sequence or the exponential growth models in population dynamics, sequences provide a powerful tool for understanding and predicting various phenomena. Their versatility and ubiquity make them an essential concept in mathematics education and scientific research.

## Notation and Writing Sequences

Understanding the formal notation for writing sequences is crucial in mathematics, particularly in the study of series and progressions. This notation provides a concise and standardized way to represent both finite and infinite sequences. The primary components of sequence notation include curly brackets, the general term, and indicators for the sequence's range.

The basic structure of a sequence in formal notation is as follows: {an}, where an represents the general term of the sequence. The subscript 'n' denotes the position of each term in the sequence. This notation is enclosed in curly brackets {} to indicate that it represents a set of numbers in a specific order.

The starting number of a sequence is typically denoted as n = 1, although this can vary depending on the context. For instance, a sequence might be written as {an}n=1, which indicates that the sequence starts at n = 1 and continues infinitely. The infinity symbol () is used to represent infinite sequences, showing that the terms continue without end.

Let's explore some examples to illustrate how to write sequences using this notation:

1. Simple arithmetic sequence: {2n}n=1
This represents the sequence 2, 4, 6, 8, 10, ..., where each term is twice the value of n.

2. Geometric sequence: {2n}n=0
This sequence is 1, 2, 4, 8, 16, ..., starting from n = 0 and doubling each term.

3. More complex formula: {3/(2n)}n=1
This sequence would be 3/2, 3/4, 3/6, 3/8, ..., where each term is 3 divided by twice n.

The general term an is particularly important as it defines the rule for generating each term in the sequence. It can be a simple expression like 2n or a more complex function. For example, in the sequence {(-1)nn}n=1, the general term involves both n and an exponent, resulting in the alternating sequence -1, 2, -3, 4, -5, ...

When writing finite sequences, we can specify the number of terms instead of using the infinity symbol. For instance, {n2}n=110 represents the first 10 square numbers: 1, 4, 9, 16, 25, 36, 49, 64, 81, 100.

Understanding and using proper sequence notation is essential for clear communication in mathematics. It allows for precise description of patterns and behaviors in number sequences, which is fundamental in various areas of mathematics, including calculus, number theory, and discrete mathematics.

In conclusion, mastering the formal notation for writing sequences enhances one's ability to analyze and work with mathematical patterns. Whether dealing with simple arithmetic progressions or complex mathematical series, this standardized notation provides a powerful tool for expressing and studying sequences of numbers.

## Finding Terms and General Formulas

Understanding sequences and their general formulas is a crucial skill in mathematics. This section will demonstrate how to find specific terms in a sequence using the general formula, calculate the first few terms, and derive the general formula from a given sequence. We'll emphasize the importance of pattern recognition throughout this process.

To find specific terms in a sequence given the general formula, we simply substitute the term number into the formula. For example, consider the arithmetic sequence with the general formula an = 3n + 2, where n is the term number. To find the 5th term, we substitute n = 5:

a5 = 3(5) + 2 = 15 + 2 = 17

Similarly, for the 10th term:

a10 = 3(10) + 2 = 30 + 2 = 32

Calculating the first few terms of a sequence helps us understand its pattern. Let's use the same formula to find the first five terms:

• a1 = 3(1) + 2 = 5
• a2 = 3(2) + 2 = 8
• a3 = 3(3) + 2 = 11
• a4 = 3(4) + 2 = 14
• a5 = 3(5) + 2 = 17

Now, let's consider the reverse process: deriving the general formula when given a sequence of terms. This requires careful pattern recognition. For instance, given the sequence 2, 6, 18, 54, 162, we can observe that each term is tripled to get the next term. This suggests a geometric sequence with a common ratio of 3.

To find the general formula, we start with the first term (a1 = 2) and multiply by 3 raised to the power of (n-1), where n is the term number. Thus, the general formula is:

an = 2 * 3n-1

We can verify this by calculating a few terms:

• a1 = 2 * 31-1 = 2 * 1 = 2
• a2 = 2 * 32-1 = 2 * 3 = 6
• a3 = 2 * 33-1 = 2 * 9 = 18

Pattern recognition is crucial in working with sequences. Look for common differences (arithmetic sequences), common ratios (geometric sequences), or more complex patterns. For example, in the sequence 1, 4, 9, 16, 25, we recognize these as perfect squares. The general formula would be an = n2.

Sometimes, sequences may have more intricate patterns. Consider 1, 3, 6, 10, 15. By looking at the differences between consecutive terms (2, 3, 4, 5), we can deduce that this is a triangular number sequence. Its general formula is an = n(n+1)/2.

In conclusion, working with sequences involves a interplay between specific terms and general formulas. By practicing these skills and honing your pattern recognition abilities, you'll become proficient at analyzing and generating sequences, a valuable skill in various mathematical and real-world applications.

## Limits of Sequences

In mathematics, the concept of limits in sequences is fundamental to understanding the behavior of infinite series and forms the basis for many advanced calculus concepts. A sequence is an ordered list of numbers that follows a specific pattern or rule. The limit of a sequence represents the value that the terms of the sequence approach as we move further along the sequence, typically as we approach infinity.

The notation for limits of sequences as n approaches infinity is typically written as: lim(n) a_n = L This notation means that as n (the term number) gets arbitrarily large, the sequence a_n approaches the limit L.

Sequences can be classified into two main categories: convergent and divergent sequences. A convergent sequence is one where the terms approach a specific finite value as n approaches infinity. In other words, the limit exists. For example, the sequence 1/n converges to 0 as n approaches infinity. We can write this as: lim(n) 1/n = 0 As n gets larger, 1/n gets closer and closer to 0.

On the other hand, a divergent sequence is one that does not have a finite limit. This can occur in two ways: either the sequence grows without bound, or it oscillates without settling on a particular value. An example of a divergent sequence that grows without bound is n^2. As n increases, n^2 grows infinitely large: lim(n) n^2 = An example of an oscillating divergent sequence is (-1)^n, which alternates between 1 and -1 indefinitely, never settling on a single value.

The squeeze theorem, also known as the sandwich theorem or pinching theorem, is a powerful tool for determining the limit of a sequence when it's difficult to calculate directly. The theorem states that if we have three sequences a_n, b_n, and c_n, where: a_n b_n c_n for all n greater than some value N, and lim(n) a_n = lim(n) c_n = L Then, lim(n) b_n must also equal L.

To apply the squeeze theorem to sequences, we "sandwich" the sequence we're interested in between two other sequences whose limits we can easily determine. If these outer sequences converge to the same limit, then our middle sequence must also converge to that limit. This theorem is particularly useful when dealing with complex sequences or those involving trigonometric functions.

For example, consider the sequence (n * sin(1/n)). We know that -1 sin(x) 1 for all x. Multiplying by n (which is always positive for n > 0), we get: -n n * sin(1/n) n Dividing all parts by n: -1 sin(1/n) 1 As n approaches infinity, both -1/n and 1/n approach 0. Therefore, by the squeeze theorem: lim(n) sin(1/n) = 0

Understanding limits of sequences is crucial for many areas of mathematics, including calculus, analysis, and even applied fields like physics and engineering. By mastering the concepts of convergence, divergence, and tools like the squeeze theorem, students can tackle more complex problems and gain deeper insights into the behavior of mathematical functions and series.

## Properties and Theorems of Sequence Limits

Sequence limits play a crucial role in mathematical analysis, providing the foundation for understanding convergence and divergence in infinite series. This section explores essential properties and theorems related to sequence limits, focusing on limit rules, absolute value sequences, and exponential sequences.

One of the fundamental aspects of sequence limits is the set of limit properties that allow us to manipulate and combine sequences. These properties include the sum rule, difference rule, product rule for sequence limits, and quotient rule for sequence limits. The sum rule states that the limit of the sum of two sequences is equal to the sum of their individual limits, provided both limits exist. Mathematically, if lim(a_n) = A and lim(b_n) = B, then lim(a_n + b_n) = A + B. Similarly, the difference rule asserts that lim(a_n - b_n) = A - B.

The product rule for sequence limits states that the limit of the product of two sequences is equal to the product of their individual limits, again assuming both limits exist. This can be expressed as lim(a_n * b_n) = A * B. The quotient rule for sequence limits, which is slightly more complex, states that if lim(a_n) = A and lim(b_n) = B, with B 0, then lim(a_n / b_n) = A / B. These properties are invaluable tools for evaluating limits of complex sequences by breaking them down into simpler components.

Another important theorem in the study of sequence limits is the absolute value sequence theorem. This theorem states that if the limit of a sequence (a_n) exists and equals L, then the limit of the absolute value of the sequence |a_n| also exists and equals |L|. Mathematically, if lim(a_n) = L, then lim(|a_n|) = |L|. This theorem is particularly significant because it allows us to relate the convergence of a sequence to the convergence of its absolute value, which can be easier to analyze in some cases.

The absolute value sequence theorem has several important implications. It provides a method for proving that certain sequences do not converge by showing that their absolute values do not converge. Additionally, it helps in establishing the relationship between the convergence of a sequence and the convergence of related sequences, such as those obtained by applying continuous functions to the original sequence.

One of the most important theorems in the study of sequence limits is the theorem for limits of exponential sequences. This theorem deals with sequences of the form (x^n), where x is a real number and n is the sequence index. The theorem states that:

1. If |x| < 1, then lim(x^n) = 0 as n approaches infinity.
2. If x = 1, then lim(x^n) = 1 as n approaches infinity.
3. If x = -1, then (x^n) oscillates between 1 and -1 and does not converge.
4. If |x| > 1, then the sequence diverges (approaches infinity or negative infinity, depending on the sign of x).

This theorem has profound implications for understanding the behavior of exponential sequences and their convergence properties. It provides a clear criterion for determining whether an exponential sequence converges or diverges based solely on the base value x. This result is extensively used in various areas of mathematics, including calculus, analysis, and applied mathematics.

The exponential sequence theorem also serves as a foundation for more advanced concepts in mathematical analysis. For instance, it is crucial in the study of power series and their convergence, as well as in the analysis of exponential and logarithmic functions. The theorem's implications extend to practical applications in fields such as physics, engineering, and economics, where exponential growth or decay processes are frequently encountered.

Understanding these properties and theorems is essential for developing a strong foundation in mathematical analysis. They provide powerful tools for evaluating limits, proving convergence and divergence of sequences, and analyzing complex mathematical expressions. By mastering these concepts, students and researchers can tackle more advanced topics in calculus, real analysis, and beyond.

## Conclusion

In this lesson, we've explored the fundamental concepts of sequences, a crucial aspect of mathematical foundations. We began with the introduction video, which laid the groundwork for understanding sequences. We then delved into the definition of sequences, learning how to represent them using proper notation. The lesson covered methods for finding specific terms within a sequence and deriving general formulas to describe entire sequences. We also touched on the concept of limits, which plays a significant role in advanced sequence analysis. It's essential to practice working with sequences to solidify your understanding. We encourage you to explore various mathematical contexts where sequences appear, such as number theory, calculus, and real-world applications. By mastering these concepts, you'll be well-equipped to tackle more complex mathematical challenges. Remember, sequences are a powerful tool in mathematics, and their applications extend far beyond this introductory lesson.

### Introduction to Sequences

Overview:
Notation of Sequences

#### Step 1: Understanding What a Sequence Is

Before diving into the notation of sequences, it's essential to understand what a sequence is. A sequence is a list of numbers that follow a specific pattern. In high school, you might have encountered arithmetic sequences and geometric sequences. These are the types of sequences we will be dealing with. Formally, a sequence is a list of numbers denoted as a1, a2, ..., an-1, an, where a1 is the first term, a2 is the second term, and so on, until an, which is the nth term of the sequence.

#### Step 2: Writing Sequences in a Formal Way

Now that we know what sequences are, the next step is to understand how to write them formally. Instead of listing all the numbers, we can use a more compact notation. One common way to write a sequence is by using the notation an, where n starts from 1 and goes to infinity. This can be written as an with curly brackets, indicating that the sequence continues indefinitely. The value of an represents the value at the nth term, and there is usually a formula associated with it, such as n2 or 4n. This formula allows us to find the value of any term in the sequence by plugging in the value of n.

#### Step 3: Examples of Sequence Notation

Let's look at some examples to understand this better. Consider the sequence denoted by {2n} where n starts from 1 and goes to infinity. To find the terms of this sequence, we plug in the values of n into the formula 2n. For n=1, the first term is 2*1=2. For n=2, the second term is 2*2=4. For n=3, the third term is 2*3=6, and so on. The sequence continues as 2, 4, 6, 8, 10, and so forth, indefinitely.

#### Step 4: Working with More Complex Formulas

Sequences can also be represented using more complex formulas. For example, consider the sequence given by the formula 3/(2n). To find the terms of this sequence, we again plug in the values of n. For n=1, the first term is 3/(2*1)=3/2. For n=2, the second term is 3/(2*2)=3/4. For n=3, the third term is 3/(2*3)=3/6, and so on. The sequence continues as 3/2, 3/4, 3/6, 3/8, and so forth. Each term can be denoted as a1, a2, a3, a4, and so on, where an represents the nth term of the sequence.

#### Step 5: Infinite Sequences

Many sequences are infinite, meaning they continue indefinitely. In the notation {an}, the infinity symbol indicates that the sequence does not have an end. For example, the sequence {2n} starts at n=1 and continues forever, producing terms like 2, 4, 6, 8, 10, and so on. Similarly, the sequence {3/(2n)} also continues indefinitely, producing terms like 3/2, 3/4, 3/6, 3/8, and so forth. Understanding that sequences can be infinite is crucial for working with them in various mathematical contexts.

#### Step 6: Practical Applications

Sequences are not just theoretical constructs; they have practical applications in various fields such as computer science, finance, and engineering. For instance, in computer science, sequences are used in algorithms and data structures. In finance, sequences can represent periodic payments or interest calculations. Understanding the notation and properties of sequences is essential for applying them effectively in these fields.

#### Conclusion

In summary, sequences are lists of numbers that follow a specific pattern, and they can be represented using various notations. The notation {an} is a compact way to represent sequences, where an denotes the value at the nth term. Sequences can be finite or infinite, and they have practical applications in various fields. By understanding the notation and properties of sequences, you can work with them more effectively in both theoretical and practical contexts.

### FAQs

#### 1. What is the difference between an arithmetic and a geometric sequence?

An arithmetic sequence has a constant difference between consecutive terms, while a geometric sequence has a constant ratio between consecutive terms. For example, 2, 5, 8, 11 is arithmetic (common difference of 3), and 2, 6, 18, 54 is geometric (common ratio of 3).

#### 2. How do you find the nth term of an arithmetic sequence?

The nth term of an arithmetic sequence is given by the formula an = a1 + (n - 1)d, where a1 is the first term, n is the position of the term, and d is the common difference.

#### 3. What is a convergent sequence?

A convergent sequence is one where the terms approach a specific finite value (called the limit) as the number of terms increases indefinitely. For example, the sequence 1/n converges to 0 as n approaches infinity.

#### 4. How can the squeeze theorem be used to find sequence limits?

The squeeze theorem states that if an bn cn for all n greater than some value, and if lim(an) = lim(cn) = L, then lim(bn) must also equal L. This is useful for finding limits of complex sequences by "sandwiching" them between simpler sequences.

#### 5. What is the limit of the sequence (1 + 1/n)n as n approaches infinity?

The limit of the sequence (1 + 1/n)n as n approaches infinity is the mathematical constant e, approximately equal to 2.71828. This sequence is important in calculus and is often used to define the number e.

### Prerequisite Topics

Before diving into the fascinating world of sequences, it's crucial to have a solid foundation in certain mathematical concepts. One of the most important prerequisite topics for understanding sequences is the Squeeze theorem. This fundamental principle plays a significant role in analyzing and determining the behavior of sequences, making it an essential tool in your mathematical toolkit.

The Squeeze theorem, also known as the sandwich theorem or pinching theorem, is a powerful concept that helps us determine the limit of a sequence by comparing it to two other sequences. This theorem is particularly useful when dealing with complex sequences where direct calculation of the limit might be challenging or impossible.

Understanding the Squeeze theorem for sequences provides a strong foundation for grasping more advanced concepts in sequence analysis. It allows you to approach problems from different angles and often simplifies the process of finding limits. This theorem is not only applicable to sequences but also extends to functions and series, making it a versatile tool in calculus and mathematical analysis.

When studying sequences, you'll frequently encounter situations where the behavior of a sequence is not immediately apparent. The Squeeze theorem comes to the rescue in such scenarios, allowing you to bound the sequence between two simpler sequences whose limits are known or easier to calculate. This approach is particularly valuable when dealing with oscillating or irregularly behaving sequences.

Moreover, the Squeeze theorem helps develop critical thinking and problem-solving skills. It encourages you to think creatively about how to bound a sequence and identify appropriate comparison sequences. These skills are invaluable not only in studying sequences but also in tackling more advanced mathematical concepts and real-world applications.

As you progress in your study of sequences, you'll find that the Squeeze theorem becomes an indispensable tool. It will help you in proving convergence, finding limits, and understanding the behavior of complex sequences. The theorem's applications extend beyond just sequences, making it a fundamental concept in calculus and mathematical analysis.

By mastering the Squeeze theorem, you'll be well-prepared to tackle more advanced topics in sequence analysis. It will provide you with a solid foundation for understanding convergence, divergence, and the limiting behavior of sequences. This knowledge will prove invaluable as you delve deeper into the study of series, infinite sums, and other advanced mathematical concepts.

In conclusion, a thorough understanding of the Squeeze theorem is essential for anyone looking to master the intricacies of sequences. It serves as a bridge between basic algebraic concepts and more advanced calculus topics, making it an indispensable prerequisite for your journey into the world of sequences and beyond.

Note:
1. If a sequence has the limit $L$, then we can say that:

$\lim$n →$\infty$ $a$$n$$=L$

If the limit is finite, then it is convergent. Otherwise, it is divergent.

2. If the limit of the sequences {$a_n$} and {$b_n$} are finite and $c$ is constant, then we can say that

i) $\lim$n →$\infty$ $(a_n+b_n)=\lim$n →$\infty$ $a_n+$$\lim$n →$\infty$ $b_n$.
ii) $\lim$n →$\infty$ $(a_n-b_n)=\lim$n →$\infty$ $a_n-$$\lim$n →$\infty$ $b_n$.
iii) $\lim$n →$\infty$ $ca_n=c$ $\lim$n →$\infty$ $a_n$.
iv) $\lim$n →$\infty$$(a_nb_n)=$ $\lim$n →$\infty$$a_n*$ $\lim$n →$\infty$ $b_n$.
v) $\lim$n →$\infty$ $[a_n$$\div$$b_n]$ $=\lim$n →$\infty$$a_n$$\div$ $\lim$n →$\infty$$b_n$$,$$b_n\neq0$.

3. If $a_n\leq c_n\leq b_n$ and $\lim$n →$\infty$ $a_n=$ $\lim$n →$\infty$ $b_n=L$, then $\lim$n →$\infty$ $c_n=L$.

4.if $\lim$n →$\infty$ $|a_n|=0$, then $\lim$n →$\infty$ $a_n=0$ as well.

5. We say that:

Where the sequence {$x^n$} is convergent for -1< $x \leq$ 1, and divergent if $x$ > 1.