Chapter 9: Problem 35
(a) Find the Maclaurin series for \(e^{x^{4}} .\) What is the radius of convergence? (b) Explain two different ways to use the Maclaurin series for \(e^{x^{4}}\) to find a series for \(x^{3} e^{x^{4}} .\) Confirm that both methods produce the same series.
Short Answer
Expert verified
The Maclaurin series for \( e^{x^4} \) is \( \sum_{n=0}^{\infty} \frac{x^{4n}}{n!} \) with an infinite radius. For \( x^3 e^{x^4} \), the series is \( \sum_{n=0}^{\infty} \frac{x^{4n+3}}{n!} \) by both methods.
Step by step solution
01
Understand the Maclaurin Series
The Maclaurin series for a function is given by the formula: \[ f(x) = \sum_{n=0}^{\infty} \frac{f^{(n)}(0)}{n!} x^n \] For the exponential function \( e^{x} \), the Maclaurin series is: \[ e^{x} = \sum_{n=0}^{\infty} \frac{x^n}{n!} \] To find the series for \( e^{x^4} \), we replace \( x \) with \( x^4 \) in the exponential series.
02
Find the Series for \( e^{x^4} \)
Replace \( x \) with \( x^4 \) in the Maclaurin series for \( e^{x} \): \[ e^{x^4} = \sum_{n=0}^{\infty} \frac{(x^4)^n}{n!} = \sum_{n=0}^{\infty} \frac{x^{4n}}{n!} \] Thus, the Maclaurin series for \( e^{x^4} \) is \( \sum_{n=0}^{\infty} \frac{x^{4n}}{n!} \).
03
Determine the Radius of Convergence
The series for \( e^{x^4} \) is a geometric series with \( x \) replaced by \( x^4 \). The radius of convergence for \( e^x \) is infinite, meaning it converges for all \( x \). Similarly, the series for \( e^{x^4} \) converges for all real \( x \), so its radius of convergence is also infinite.
04
Approach 1 to Find Series for \( x^3 e^{x^4} \)
To find the series for \( x^3 e^{x^4} \), multiply the entire series for \( e^{x^4} \) by \( x^3 \):\[ x^3 \times e^{x^4} = x^3 \sum_{n=0}^{\infty} \frac{x^{4n}}{n!} = \sum_{n=0}^{\infty} \frac{x^{4n+3}}{n!} \] This multiplication results in the series \( \sum_{n=0}^{\infty} \frac{x^{4n+3}}{n!} \).
05
Approach 2 to Find Series for \( x^3 e^{x^4} \)
Consider \( x^3 e^{x^4} \) as \( f(x) \cdot g(x) \) where \( f(x) = x^3 \) and \( g(x) = e^{x^4} \). Using the series for \( e^{x^4} \), evaluate \( x^3 \, g(x) \):\[ g(x) = \sum_{n=0}^{\infty} \frac{x^{4n}}{n!} \rightarrow f(x)g(x) = x^3 \sum_{n=0}^{\infty} \frac{x^{4n}}{n!} = \sum_{n=0}^{\infty} \frac{x^{4n+3}}{n!} \] Both approaches result in the series \( \sum_{n=0}^{\infty} \frac{x^{4n+3}}{n!} \).
06
Confirm Both Methods Produce the Same Series
Both methods yield the series \( \sum_{n=0}^{\infty} \frac{x^{4n+3}}{n!} \). By multiplying the original Maclaurin series for \( e^{x^4} \) by \( x^3 \) directly and treating \( x^3 e^{x^4} \) as \( f(x) \cdot g(x) \), we've confirmed the consistency of the result.
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Key Concepts
These are the key concepts you need to understand to accurately answer the question.
Exponential function
An exponential function is a mathematical expression where a constant base is raised to a variable exponent. Among all exponential functions, one of the most important is the natural exponential function, denoted as \( e^x \). This function has unique properties, such as its own derivative being itself.In the context of the Maclaurin series, the exponential function \( e^x \) can be expanded into an infinite series:
- This series is expressed as \( e^x = \sum_{n=0}^{\infty} \frac{x^n}{n!} \).
- Each term of this series consists of \( x \) raised to the power of \( n \), divided by the factorial of \( n \).
Radius of convergence
The radius of convergence in an infinite series is a measure of the range of values that \( x \) can take for which the series converges to a definite value. For the exponential series \( e^x \), the radius of convergence is infinite. This means:
- The series converges for all real numbers \( x \), without any restriction.
- This property makes the exponential function particularly useful since it applies across the entire set of real numbers.
Infinite series
An infinite series is essentially a sum of infinitely many terms. It allows us to express functions that might not be immediate to evaluate or understand in a closed form. This is especially true for exponential functions and other transcendental functions.The Maclaurin series representation is a type of infinite series that originates by using derivatives at zero. For example, for \( e^x \), it yields:
- \( \sum_{n=0}^{\infty} \frac{x^n}{n!} \).
- The series provides a way to approximate functions using a sum of simple polynomial terms.