 # Alternating Series Estimation Theorem – Definition With Examples

##  The Alternating Series Estimation Theorem is a powerful tool in mathematics, offering us remarkable insights into the dynamics of alternating series.

This theorem guides approximating the sum of an alternating series, serving as a critical component in understanding convergent series and real analysis. The article aims to decode this theorem, making it more approachable for mathematics enthusiasts.

Whether you’re a seasoned researcher, a curious student, or just a seeker of mathematical knowledge, this comprehensive examination of the Alternating Series Estimation Theorem will give you an immersive dive into the subject, illuminating its nuances and importance in the broader mathematical landscape.

## Definition of Alternating Series Estimation Theorem

The Alternating Series Estimation Theorem is a mathematical theorem within calculus and real analysis. It’s a principle used to estimate the value of a series that alternates in sign. Specifically, the theorem applies to a series that fits the following two conditions:

1. Each term in the series is less than or equal to the term before it: aₙ₊₁ ≤ aₙ.
2. The limit of the terms as n approaches infinity is zero: lim (n→∞) aₙ = 0.

The theorem states that for an alternating series satisfying these conditions, the absolute value of the difference between the sum of the series and the sum of the first n terms is less than or equal to the absolute value of the (n+1)th term.

In simpler terms, it provides an upper bound for the error when approximating the sum of the entire series by the sum of the first n terms. It’s a valuable tool for making sense of infinite series and approximating their sums, which can be particularly useful in scientific, engineering, and statistical contexts.

## Historical Significance

The roots of the theorem can be traced back to the work of early mathematicians in ancient Greece, notably Zeno of Elea, who proposed several paradoxes related to infinite series. This work was significantly expanded upon in the late Middle Ages and early Renaissance when European mathematicians began to grapple with infinity more rigorously and formally.

However, the real development of the formal theory of series, including alternating series, did not occur until the invention of calculus by Isaac Newton and Gottfried Wilhelm Leibniz in the 17th century.

This work was later formalized and made rigorous by Augustin-Louis Cauchy in the 19th century, who developed the modern definition of a limit and used it to prove many results about series, including alternating series.

The Alternating Series Estimation Theorem is a relatively straightforward consequence of these more general results about series and convergence, and it’s not associated with any specific mathematician or moment in history. Its simplicity and usefulness, however, have made it an important part of the standard curriculum in calculus and real analysis.

So while the Alternating Series Estimation Theorem doesn’t have a single, clear historical origin, it’s a product of centuries of mathematical thought and investigation into the nature of infinity and the behavior of infinite series.

## Properties

The Alternating Series Estimation Theorem is defined by two primary properties, also known as conditions or criteria, which need to be met for the theorem to apply:

### Decreasing the Magnitude of Terms

The absolute values of the terms in the series need to be monotonically decreasing. This means each term in the series should be less than or equal to the previous term. Mathematically, it can be stated as aₙ₊₁ ≤ aₙ for all n. Essentially, the sizes of the terms are getting progressively smaller.

### Limit of Terms Approaches Zero

The limit of the terms in the series as n approaches infinity should be zero. Formally, this is written as lim (n→∞) aₙ = 0. This means that as you move farther and farther along the series, the terms get closer and closer to zero.

If these two conditions are met, the series is known as a convergent alternating series, and the Alternating Series Estimation Theorem can be applied.

The theorem then estimates the error when approximating an alternating series sum. It states that if S is the sum of the infinite series and Sₙ is the sum of the first n terms of the series, then the absolute error |S – Sₙ| is less than or equal to the absolute value of the next term aₙ₊₁. This allows us to bind the error when we only sum the first n terms of an infinite alternating series.

## Applications

The Alternating Series Estimation Theorem finds diverse applications in various fields due to its utility in approximating infinite series, particularly those with alternating terms. Below are a few examples of where this theorem can be applied:

### Computer Science

In computer science, especially in areas like algorithmic analysis, alternating series can model the behavior of computational processes. The theorem can be used to estimate errors and approximate results.

### Physics

Physics often involves models and calculations with infinite series. For instance, some wave functions are expressed as infinite series in quantum mechanics. The Alternating Series Estimation Theorem can help give a good approximation of these functions or help estimate the error of an approximation.

### Engineering

In engineering, the theorem can be used in signal processing where Fourier series (which can be alternating) are commonly used. It can also be used in control theory to analyze the stability of control systems.

### Economics and Finance

In economics and finance, alternating series can appear in net present value calculations for cash flows or alternating payments. The theorem can be used to estimate the total value.

### Mathematical Analysis

Of course, within mathematics itself, the theorem is an important tool in real and complex analysis. It helps estimate the convergence of alternating series, which is ubiquitous in mathematics.

### Numerical Methods

In numerical methods, the theorem can be used to approximate values of functions and to estimate the speed of convergence of series solutions to differential equations.

## Exercise

### Example 1

Estimate the value of the series: S = 1 – 1/2 + 1/3 – 1/4 + 1/5 – 1/6 + …

### Solution

To find the sum of the first four terms (S₄), we get:

S₄ = 1 – 1/2 + 1/3 – 1/4

S₄ = 0.583333

According to the Alternating Series Estimation Theorem, the error |S – S₄| is less than or equal to the absolute value of the next term:

a₅ = 1/5

a₅ = 0.2.

### Example 2

Estimate the value of the series: S = 1 – 1/4 + 1/9 – 1/16 + 1/25 – 1/36 + …

### Solution

The sum of the first four terms (S₄) is:

S₄ = 1 – 1/4 + 1/9 – 1/16

S₄ = 0.597222

According to the Alternating Series Estimation Theorem, the error |S – S₄| is less than or equal to the absolute value of the next term:

a₅ = 1/25

a₅ = 0.04.

### Example 3

Estimate the value of the series: S = 1 – 1/3 + 1/5 – 1/7 + 1/9 – 1/11 + …

### Solution

The sum of the first four terms (S₄) is:

S₄ = 1 – 1/3 + 1/5 – 1/7

S₄ = 0.67619.

According to the Alternating Series Estimation Theorem, the error |S – S₄| is less than or equal to the absolute value of the next term:

a₅  = 1/9

a₅ = 0.1111

### Example 4

Estimate the value of the series: S = 1/2 – 1/4 + 1/6 – 1/8 + 1/10 – 1/12 + …

### Solution

The sum of the first four terms (S₄) is:

S₄ = 1/2 – 1/4 + 1/6 – 1/8

S₄ = 0.291667

According to the Alternating Series Estimation Theorem, the error |S – S₄| is less than or equal to the absolute value of the next term:

a₅  = 1/10

a₅ = 0.1

### Example 5

Estimate the value of the series: S = 1/3 – 1/9 + 1/15 – 1/21 + 1/27 – 1/33 + …

### Solution

The sum of the first four terms (S₄) is:

S₄ = 1/3 – 1/9 + 1/15 – 1/21

S₄ = 0.165343

According to the Alternating Series Estimation Theorem, the error |S – S₄| is less than or equal to the absolute value of the next term:

a₅ = 1/27

a₅ = 0.03704

### Example 6

Estimate the value of the series: S = 1 – $(1/2)^2$ + $(1/3)^2$ – $(1/4)^2$ + $(1/5)^2$ – $(1/6)^2$ + …

### Solution

The sum of the first four terms (S₄) is:

S₄ =  1 – $(1/2)^2$ + $(1/3)^2$ – $(1/4)^2$

S₄ = 0.854167

According to the Alternating Series Estimation Theorem, the error |S – S₄| is less than or equal to the absolute value of the next term:

a₅ = $(1/5)^2$

a₅ = 0.04

### Example 7

Estimate the value of the series: S = 1/4 – 1/16 + 1/36 – 1/64 + 1/100 – 1/144 + …

### Solution

The sum of the first four terms (S₄) is:

S₄ = 1/4 – 1/16 + 1/36 – 1/64

S₄ = 0.208333.

According to the Alternating Series Estimation Theorem, the error |S – S₄| is less than or equal to the absolute value of the next term:

a₅ = 1/100

a₅ = 0.01

### Example 8

Estimate the value of the series: S = 1/5 – 1/25 + 1/45 – 1/65 + 1/85 – 1/105 + …

### Solution

The sum of the first four terms (S₄) is:

S₄ = 1/5 – 1/25 + 1/45 – 1/65

S₄ = 0.171154

According to the Alternating Series Estimation Theorem, the error |S – S₄| is less than or equal to the absolute value of the next term:

a₅ = 1/85

a₅ = 0.011764