# How to find interval of convergence?

- Author: Noreen Niazi
- Last Updated on: January 21, 2024

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ToggleStudying about power series in mathematics requires understanding the interval of convergence. Whether you are a student taking the plunge into calculus or an old-timer in the world of mathematics, this article hopes to deconstruct the process of finding convergence. 10 solved examples and 5-word problems will give us practical insights into the mathematical intricacies of this topic while covering the theoretical backgrounds.

## I. Theoretical Foundations:

Before we proceed to solving problems, let us first establish the basics.**A. Power Series:**

A power series is an infinite summation of terms and usually written as ∑(aₙxⁿ) where a_n denotes the coefficient, while x denote the variable.**B. Convergence of a Series:**

A series of powers is convergent if the terms get closer to 0 as n goes towards infinity. To identify the interval of convergence, we consider the values x for which there is convergence.

II. Finding Interval of Convergence:

**A. Ratio Test:**

Ratio test is good for evaluating infinite series. If the Ratio Test is applied to determine a interval of convergence, evaluate lim(│aₙ+₁/aₙ || as n ≈ infty.**B. Root Test:**

Another method of determining the convergence of series is the Root Test. Determine lim(│aₙ│^(1/n)) and find the interval of convergence based on it.**C. Endpoints Analysis:**

Assess the convergence at the ends of segment to determine precision. This includes the investigation of series behavior at x values on boundary points of interval.

## How to find interval of convergence? Solved Examples

**Example 1: ∑(n!xⁿ/nⁿ)****Solution:**Apply the Ratio Test to determine the interval of convergence.**Ratio Test:**$$\underset{n\to \mathrm{\infty}}{\mathrm{lim}}\mid \frac{{\mathrm{an}+1}_{}}{{a}_{n}}\mid =\underset{n\to \mathrm{\infty}}{\mathrm{lim}}\mid \frac{(n+1)!{x}^{n+1}\mathrm{/}(n+1{)}^{n+1}}{n!{x}^{n}\mathrm{/}{n}^{n}}\mid $$**Simplify:**$$\underset{n\to \mathrm{\infty}}{\mathrm{lim}}\mid \frac{x(n+1)}{n+1}\mid $$**Limit Calculation:**$$\underset{n\to \mathrm{\infty}}{\mathrm{lim}}\mathrm{\mid}x\mathrm{\mid}=\mathrm{\mid x}\mathrm{\mid}$$**Interval of Convergence:**- The series converges absolutely when $\mathrm{\mid}x\mathrm{\mid}<1$.
- Thus, the interval of convergence is $(-1,1)$.

**Example 2: ∑(2ⁿxⁿ/n!)****Solution:**Apply the Ratio Test to find the interval of convergence.**Ratio Test:**$$\underset{n\to \mathrm{\infty}}{\mathrm{lim}}\mid \frac{{a}_{n+1}}{{a}_{n}}\mid =\underset{n\to \mathrm{\infty}}{\mathrm{lim}}\mid \frac{{2}^{n+1}{x}^{n+1}\mathrm{/}(n+1)!}{{2}^{n}{x}^{n}\mathrm{/}n!}\mid $$**Simplify:**$$\underset{n\to \mathrm{\infty}}{\mathrm{lim}}\mid \frac{2x}{n+1}\mid $$**Limit Calculation:**$$\underset{n\to \mathrm{\infty}}{\mathrm{lim}}\frac{2x}{n+1}=0$$**Interval of Convergence:**- The series converges for all values of.
- Therefore, the interval of convergence is $(-\mathrm{\infty},\mathrm{\infty})$.

**Example 3: ∑(sin(nx)/n)xⁿ****Solution:**Apply the Ratio Test, considering the alternating series and the behavior of the sine function.**Example 4: ∑(n^2xⁿ)****Solution:**Apply the Ratio Test to examine the behavior of the series concerning $$.**Example 5: ∑(xⁿ/n^2)****Solution:**Use the Ratio Test to determine the interval of convergence for this series.Remember to adapt the steps based on the characteristics of each series, such as alternating signs, factorial terms, or exponential growth.

## Word Problems:

**A company’s profit is modeled by ∑(1000xⁿ/n²). Find where this model is valid.****Solution:**Apply the Ratio Test:This limit converges for all x , indicating the model is valid for all x.

**An investment grows according to ∑(2ⁿxⁿ/3ⁿ). Determine the growth interval.****Solution:**Apply the Ratio Test:$$\underset{n\to \mathrm{\infty}}{\mathrm{lim}}\mid \frac{{2}^{n+1}{x}^{n+1}\mathrm{/}{3}^{n+1}}{{2}^{n}{x}^{n}\mathrm{/}{3}^{n}}\mid =\underset{n\to \mathrm{\infty}}{\mathrm{lim}}\mid \frac{x}{3}\mid $$This limit converges when∣x∣<3,, indicating the investment grows for(−3,3).

**A population is modeled by ∑(xⁿ/n!). Identify where the population model converges.****Solution:**Apply the Ratio Test:The limit converges for all x, indicating the model is valid for all x.

**A radioactive substance decays with ∑((-1)ⁿxⁿ/n). Discover where the decay is valid.****Solution:**Apply the Ratio Test:The limit converges for all x, indicating the decay model is valid for all x.

**A ball’s height follows ∑(5ⁿxⁿ/n²). Determine when this series converges.****Solution:**Apply the Ratio Test:$$\underset{n\to \mathrm{\infty}}{\mathrm{lim}}\mid \frac{{5}^{n+1}{x}^{n+1}\mathrm{/}(n+1{)}^{2}}{{5}^{n}{x}^{n}\mathrm{/}{n}^{2}}\mid =\underset{n\to \mathrm{\infty}}{\mathrm{lim}}\mid \frac{x}{n+1}\mid $$The limit converges for all x, indicating the ball’s height series converges for all x.

## Real-World Applications:

The interpretation of the interval of convergence has many practical implications which go beyond pure mathematics. Let us now look at some cases in which these notions find applications.

**Economics:**

The common use of power series in economic models includes representation factors such as inflation, interest rates and growth. The interval of convergence helps economists identify the range for which these models are valid.

**Physics:**

Power series describe physical phenomena, like wave functions and particle behavior. To make reliable predictions and interpretations in physics, specifying the interval of convergence is an important prerequisite for these series.

**Computer Science:**

Both algorithms and computational methods use mathematical models that often involve power series. While the convergence period is established, these models are reliable in computer simulations and data analysis.

**Engineering:**

Engineers use the power series in modeling signals, vibrations, and other dynamic systems. The ability to determine the interval of convergence should be deemed critical in building problem designs and optimization processes for engineering physics.

## Exploring Further:

To deepen your understanding of finding the interval of convergence, consider exploring additional topics and related concepts:

**The radius of Convergence:**

Learn how to identify the radius of convergence for a power series so that you know in which area the series converges.

**Taylor and Maclaurin Series:**

Get to know the Taylor and Maclaurin series, which a power expansion of functions. The convergence properties are important to understand their use.**Complex Analysis:**

Broaden your horizon to complex numbers and learn how power series behave in the real plane. In complex analysis, the notion of radius of convergence is very significant.**Applications in Differential Equations:**

Investigate how power series solutions can help solve differential equations. This application connects the dots between calculus and differential equations.

## Practice Problems

Determine the interval of convergence for the series

${\sum}_{n=0}^{\mathrm{\infty}}\frac{{x}^{n}}{{n}^{2}}$Find the interval of convergence for the series

${\sum}_{n=1}^{\mathrm{\infty}}\frac{(-1{)}^{n}{x}^{n}}{{n}^{3}}$Investigate the interval of convergence for the series

${\sum}_{n=0}^{\mathrm{\infty}}\frac{{2}^{n}{x}^{n}}{n!}$Determine the interval of convergence for the series

${\sum}_{n=1}^{\mathrm{\infty}}\frac{(-3{)}^{n}{x}^{2n}}{n}$Find the interval of convergence for the series

${\sum}_{n=0}^{\mathrm{\infty}}\frac{(4x{)}^{n}}{n!}$Investigate the convergence interval for the series

${\sum}_{n=1}^{\mathrm{\infty}}\frac{(-1{)}^{n}\cdot {2}^{n}{x}^{2n}}{{n}^{2}}$Determine the interval of convergence for the series

${\sum}_{n=0}^{\mathrm{\infty}}\frac{{x}^{2n+1}}{2n+1}$Find the interval of convergence for the series

${\sum}_{n=0}^{\mathrm{\infty}}\frac{(-2x{)}^{n}}{{3}^{n}}$Investigate the convergence interval for the series

${\sum}_{n=1}^{\mathrm{\infty}}\frac{(-1{)}^{n}{x}^{3n}}{{n}^{3}}$Determine the interval of convergence for the series

${\sum}_{n=0}^{\mathrm{\infty}}\frac{(2x-1{)}^{n}}{{2}^{n}}$

## Answer Keys of practice Problems

- $(-1,1)$
- $(-1,1)$
- $(-\mathrm{\infty},\mathrm{\infty})$
- $(-1\mathrm{/}3,1\mathrm{/}3)$
- $(-\mathrm{\infty},\mathrm{\infty})$
- $(-2,2)$
- $(-1,1)$
- $(-3\mathrm{/}2,3\mathrm{/}2)$
- $(-1,1)$
- $(0,2)$

## Conclusion:

## FAQs

A: Absolutely, a sequence can converge everywhere or nowhere. It relies on the particular series and its attributes.

A: Depending on the type of series, determine whether to use Ratio test or Root Test. If it is inconclusive, other methods such as endpoints analysis can be used.

A: When it comes to series, the interval of convergence may be open, closed or some combination thereof. Endpoint analysis is vital for a comprehensive view.

A: Despite the absence of a general solution, knowing how to use the Ratio Test and Root Test can streamline solving many series..

A: Flip-flopping signs may suggest an alternating sequence, which requires the convergence pattern to be determined. The Ratio Test or Root Test is still valid.

These word problems and FAQs give further information about practical usage as well as frequently asked questions on the topic finding interval of convergence. You may also delve deeper into the theory and implement these ideas in different mathematical circumstances.