For Which Positive Integers K Is The Following Series Convergent

For Which Positive Integers k is the Series Σ (n^k)/(n^2 + 1) Convergent?

Determining the convergence of the series Σ (n^k)/(n^2 + 1), where n ranges from 1 to infinity and k is a positive integer, requires a careful application of convergence tests. This seemingly simple series reveals a rich interplay between the exponent k and the behavior of the terms as n becomes large. We'll explore different approaches to solve this problem, revealing the underlying mathematical principles and offering a comprehensive understanding of the convergence criteria.

Introduction

The series we are examining is:

Σ (n^k)/(n^2 + 1) where n = 1, 2, 3, ... and k is a positive integer.

Our goal is to find the values of k for which this series converges. Convergence means that the sum of the infinite series approaches a finite limit. We'll utilize the Limit Comparison Test, a powerful tool for comparing the behavior of a given series to a known convergent or divergent series. This test will allow us to efficiently analyze the convergence based on the value of k.

Understanding the Limit Comparison Test

Before diving into the solution, let's refresh our understanding of the Limit Comparison Test. This test states that if we have two series, Σ a_n and Σ b_n, where a_n, b_n > 0 for all n, and:

lim (n→∞) (a_n / b_n) = L, where L is a finite positive number (0 < L < ∞),

then either both series converge or both series diverge. Choosing an appropriate comparison series, Σ b_n, is crucial for applying this test effectively.

Applying the Limit Comparison Test

Let's consider our series, Σ (n^k)/(n^2 + 1). For large values of n, the "+1" in the denominator becomes insignificant compared to n². Therefore, we can approximate the terms of the series as:

(n^k)/(n^2 + 1) ≈ (n^k)/n² = n^(k-2)

This suggests comparing our series to the p-series, Σ n^(k-2). The p-series, Σ (1/n^p), converges if p > 1 and diverges if p ≤ 1. In our case, p = 2 - k.

Now, let's formally apply the Limit Comparison Test:

Let a_n = (n^k)/(n² + 1) and b_n = n^(k-2).

Then,

lim (n→∞) [(n^k)/(n² + 1)] / [n^(k-2)] = lim (n→∞) [n^k / (n²(1 + 1/n²))] * [n^(2-k)]

= lim (n→∞) [n^(k+2-k)] / (1 + 1/n²) = lim (n→∞) n²/ (1 + 1/n²) = 1

Since the limit is 1 (a finite positive number), the convergence of our series, Σ a_n, is directly related to the convergence of the p-series, Σ b_n = Σ n^(k-2).

Determining the Convergence Based on k

We know that the p-series, Σ n^(k-2), converges if 2 - k > 1, which simplifies to k < 1. However, we are only considering positive integer values of k. Since k must be a positive integer, the inequality k < 1 is never satisfied for positive integers. Therefore, for all positive integer values of k, the p-series Σ n^(k-2) diverges.

Consequently, by the Limit Comparison Test, our original series, Σ (n^k)/(n² + 1), also diverges for all positive integers k.

Alternative Approach: Integral Test

We can also approach this problem using the Integral Test. The Integral Test states that if f(x) is a positive, continuous, and decreasing function on the interval [1, ∞), then the series Σ f(n) converges if and only if the improper integral ∫₁^∞ f(x) dx converges.

Let's consider the function f(x) = (x^k)/(x² + 1). For positive integers k, this function is positive and continuous on [1, ∞). It's also decreasing for sufficiently large x.

Now, let's examine the improper integral:

∫₁^∞ (x^k)/(x² + 1) dx

This integral is challenging to solve analytically for arbitrary k. However, we can analyze its behavior for large x. The integrand behaves approximately like x^(k-2). The integral of x^(k-2) diverges if k-2 ≥ -1 (k ≥ 1), which is true for all positive integers k. This indicates that the integral, and hence the series, diverges for all positive integers k.

Conclusion

Through both the Limit Comparison Test and a consideration of the Integral Test, we've conclusively shown that the series Σ (n^k)/(n² + 1) diverges for all positive integers k. The dominant term n^k in the numerator, even when compared to the n² in the denominator, ultimately leads to the series' divergence. The key insight lies in understanding how the exponent k influences the behavior of the series terms for large n, leading to a divergent p-series comparison.

Frequently Asked Questions (FAQ)

• Q: Why is the "+1" in the denominator insignificant for large n? A: As n approaches infinity, the "+1" becomes negligible compared to n². The relative contribution of "+1" diminishes, making it inconsequential in determining the convergence or divergence of the series.

• Q: Could we use the Ratio Test? A: The Ratio Test might be less efficient here. While applicable, it would lead to a limit that is difficult to evaluate directly for general k and wouldn’t provide a clear and concise answer. The Limit Comparison Test provides a more straightforward approach in this particular scenario.

• Q: What if k were not a positive integer but a real number? A: The analysis would become more complex. The conditions for the convergence of the p-series and the behavior of the integral would need to be examined across the entire real number line for k, rather than just positive integers.

• Q: Are there any other convergence tests that could be applied? A: Yes, other tests like the Direct Comparison Test could also be applied, but the Limit Comparison Test offers a cleaner and more insightful approach in this specific case.

In summary, understanding convergence tests is essential for analyzing infinite series. The careful application of the Limit Comparison Test (or the Integral Test) allows us to efficiently determine the convergence or divergence of the series Σ (n^k)/(n² + 1) for all positive integers k, leading to the conclusion that it diverges in all such cases. The analysis highlights the importance of asymptotic behavior of terms in determining the convergence of infinite series.