Which Of The Following Series Is Divergent

Divergence in mathematical series reveals fascinating insights into the behavior of infinite sums, marking a critical concept for students and professionals alike. Understanding when a series diverges is vital for various fields, including calculus, analysis, and engineering. Let's explore the criteria, tests, and examples that help determine if a series is divergent.

Understanding Series Divergence

A series is the sum of the terms of a sequence. For example, if we have a sequence a₁, a₂, a₃, ..., the corresponding series is a₁ + a₂ + a₃ + ... . A series is said to be divergent if the sum of its terms does not approach a finite limit. In simpler terms, the sum either goes to infinity, negative infinity, or oscillates without settling on a specific value.

Types of Series

Before diving into divergence, let’s briefly touch on different types of series:

• Arithmetic Series: The difference between consecutive terms is constant.
• Geometric Series: Each term is multiplied by a constant ratio to get the next term.
• Harmonic Series: The sum of the reciprocals of all positive integers.
• Power Series: An infinite series in the form of Σ cₙ(x-a)ⁿ.

Why Does Divergence Matter?

Divergence is not merely a theoretical concept; it has practical implications:

• Engineering: In signal processing, understanding divergent series helps identify unstable systems.
• Physics: In quantum mechanics, certain calculations involve infinite series; knowing when these diverge is critical for meaningful results.
• Computer Science: Analyzing the convergence or divergence of series is crucial in algorithm design and analysis.

Criteria for Divergence

Several criteria can help determine if a series diverges. These criteria often involve examining the behavior of the terms and the partial sums of the series.

1. The Divergence Test (nth-Term Test)

The most basic test for divergence is the Divergence Test, also known as the nth-Term Test.

Theorem: If lim (as n approaches infinity) of aₙ is not equal to 0, then the series Σ aₙ diverges.

This test states that if the terms of the series do not approach zero as n goes to infinity, then the series cannot converge. It’s important to note that this test can only prove divergence; it cannot prove convergence. If the limit of aₙ is 0, the series may either converge or diverge, requiring further testing.

Example:

Consider the series:

Σ (n / (n + 1)) from n=1 to infinity

Let's find the limit of the term as n approaches infinity:

lim (as n approaches infinity) of (n / (n + 1)) = 1

Since the limit is 1, which is not 0, the series diverges by the nth-Term Test.

2. Comparison Test

The Comparison Test involves comparing the given series with another series whose convergence or divergence is known.

Theorem:

• If 0 ≤ aₙ ≤ bₙ for all n, and Σ bₙ converges, then Σ aₙ also converges.
• If aₙ ≥ bₙ ≥ 0 for all n, and Σ bₙ diverges, then Σ aₙ also diverges.

In simpler terms, if our series is term-by-term smaller than a convergent series, it also converges. Conversely, if our series is term-by-term larger than a divergent series, it also diverges.

Example:

Consider the series:

Σ (1 / (n - 1)) from n=2 to infinity

We can compare this series with the harmonic series Σ (1 / n), which is known to diverge.

For n ≥ 2, (1 / (n - 1)) > (1 / n). Since the harmonic series diverges, by the Comparison Test, the series Σ (1 / (n - 1)) also diverges.

3. Limit Comparison Test

The Limit Comparison Test is a variant of the Comparison Test that is often easier to use.

Theorem: If lim (as n approaches infinity) of (aₙ / bₙ) = c, where 0 < c < ∞, then Σ aₙ and Σ bₙ either both converge or both diverge.

This test states that if the ratio of the terms of two series approaches a finite, non-zero constant, then both series behave the same way – either both converge or both diverge.

Example:

Consider the series:

Σ ((2n + 1) / (n² + 3n)) from n=1 to infinity

We can compare this series with Σ (1 / n), which is the harmonic series and is known to diverge.

Let aₙ = (2n + 1) / (n² + 3n) and bₙ = 1 / n.

lim (as n approaches infinity) of (aₙ / bₙ) = lim (as n approaches infinity) of (((2n + 1) / (n² + 3n)) / (1 / n))

= lim (as n approaches infinity) of (n(2n + 1) / (n² + 3n))

= lim (as n approaches infinity) of ((2n² + n) / (n² + 3n))

= 2

Since the limit is 2 (0 < 2 < ∞), and the harmonic series Σ (1 / n) diverges, by the Limit Comparison Test, the series Σ ((2n + 1) / (n² + 3n)) also diverges.

4. Integral Test

The Integral Test connects the convergence or divergence of a series to the convergence or divergence of an improper integral.

Theorem: If f(x) is a continuous, positive, and decreasing function for x ≥ 1, then the series Σ f(n) from n=1 to infinity and the integral ∫ f(x) dx from 1 to infinity either both converge or both diverge.

This test allows us to use integral calculus to determine the behavior of a series.

Example:

Consider the series:

Σ (1 / n) from n=1 to infinity (Harmonic Series)

Let f(x) = 1 / x. This function is continuous, positive, and decreasing for x ≥ 1.

Now, let's evaluate the improper integral:

∫ (1 / x) dx from 1 to infinity

= lim (as b approaches infinity) of ∫ (1 / x) dx from 1 to b

= lim (as b approaches infinity) of [ln|x|] from 1 to b

= lim (as b approaches infinity) of (ln(b) - ln(1))

= lim (as b approaches infinity) of ln(b)

= ∞

Since the integral diverges, by the Integral Test, the harmonic series Σ (1 / n) also diverges.

5. Ratio Test

The Ratio Test is particularly useful for series involving factorials or exponential terms.

Theorem: Let L = lim (as n approaches infinity) of |aₙ₊₁ / aₙ|

• If L < 1, then the series Σ aₙ converges absolutely.
• If L > 1 or L = ∞, then the series Σ aₙ diverges.
• If L = 1, the test is inconclusive.

This test examines the ratio of consecutive terms; if this ratio approaches a value greater than 1, the series diverges.

Example:

Consider the series:

Σ (n / 2ⁿ) from n=1 to infinity

Let's apply the Ratio Test:

aₙ = n / 2ⁿ

aₙ₊₁ = (n + 1) / 2ⁿ⁺¹

L = lim (as n approaches infinity) of |aₙ₊₁ / aₙ|

= lim (as n approaches infinity) of |((n + 1) / 2ⁿ⁺¹) / (n / 2ⁿ)|

= lim (as n approaches infinity) of |((n + 1) / 2ⁿ⁺¹) * (2ⁿ / n)|

= lim (as n approaches infinity) of |(n + 1) / (2n)|

= 1 / 2

Since L = 1 / 2 < 1, the series converges. Note that the Ratio Test helps to prove convergence, not divergence in this case.

6. Root Test

The Root Test is another useful test, particularly for series where terms involve nth powers.

Theorem: Let L = lim (as n approaches infinity) of |aₙ|^(1/n)

• If L < 1, then the series Σ aₙ converges absolutely.
• If L > 1 or L = ∞, then the series Σ aₙ diverges.
• If L = 1, the test is inconclusive.

This test examines the nth root of the absolute value of the terms; if this limit is greater than 1, the series diverges.

Example:

Consider the series:

Σ ( (n / (n + 1)) ^(n²) ) from n=1 to infinity

Let's apply the Root Test:

aₙ = (n / (n + 1)) ^(n²)

L = lim (as n approaches infinity) of |aₙ|^(1/n)

= lim (as n approaches infinity) of |( (n / (n + 1)) ^(n²) )|^(1/n)

= lim (as n approaches infinity) of (n / (n + 1)) ^(n)

= lim (as n approaches infinity) of (1 / (1 + (1 / n))) ^(n)

= 1 / e

Since L = 1 / e < 1, the series converges. Again, this test illustrates convergence rather than divergence.

Common Divergent Series

Knowing some common divergent series can provide a basis for comparison and a quick reference when analyzing more complex series.

1. Harmonic Series

The Harmonic Series is a classic example of a divergent series:

Σ (1 / n) from n=1 to infinity = 1 + (1 / 2) + (1 / 3) + (1 / 4) + ...

As demonstrated earlier using the Integral Test, this series diverges.

2. p-Series with p ≤ 1

A p-series is a series of the form:

Σ (1 / n^p) from n=1 to infinity

If p ≤ 1, the p-series diverges. The harmonic series is a special case of the p-series with p = 1.

Example:

Σ (1 / √n) from n=1 to infinity, where p = 1/2. This series diverges because p < 1.

3. Geometric Series with |r| ≥ 1

A geometric series is a series of the form:

Σ ar^(n-1) from n=1 to infinity

where a is a constant and r is the common ratio. If |r| ≥ 1, the geometric series diverges.

Example:

Σ 2ⁿ from n=1 to infinity, where r = 2. This series diverges because |2| ≥ 1.

Advanced Techniques and Considerations

While the basic tests cover a wide range of series, some series require more advanced techniques.

1. Alternating Series Test

For alternating series, which have terms that alternate in sign, the Alternating Series Test can be useful in determining convergence. However, if the conditions of this test are not met, the series might diverge.

Theorem: If aₙ is a decreasing sequence and lim (as n approaches infinity) of aₙ = 0, then the alternating series Σ (-1)ⁿ aₙ converges.

If the limit of aₙ is not 0, the alternating series diverges.

2. Dirichlet's Test

Dirichlet's Test is a more general test that can be applied to series that are not necessarily alternating but involve products of sequences.

Theorem: If Σ bₙ has bounded partial sums and aₙ is a decreasing sequence that converges to 0, then the series Σ aₙ bₙ converges.

This test is useful for more complex series where other tests may be inconclusive.

3. Abel's Test

Abel's Test is another advanced test related to Dirichlet's Test.

Theorem: If Σ bₙ converges and aₙ is a bounded monotonic sequence, then the series Σ aₙ bₙ converges.

These advanced tests are typically used in more theoretical contexts and may not be necessary for elementary analysis.

Practical Examples and Applications

Let’s delve into some practical examples to solidify our understanding of divergent series.

Example 1: Series with Factorials

Consider the series:

Σ (n! / nⁿ) from n=1 to infinity

To determine if this series diverges, we can use the Ratio Test:

aₙ = n! / nⁿ

aₙ₊₁ = (n + 1)! / (n + 1)^(n+1)

L = lim (as n approaches infinity) of |aₙ₊₁ / aₙ|

= lim (as n approaches infinity) of |((n + 1)! / (n + 1)^(n+1)) / (n! / nⁿ)|

= lim (as n approaches infinity) of |((n + 1)! / (n + 1)^(n+1)) * (nⁿ / n!)|

= lim (as n approaches infinity) of |((n + 1) * n! / (n + 1)^(n+1)) * (nⁿ / n!)|

= lim (as n approaches infinity) of |nⁿ / (n + 1)ⁿ|

= lim (as n approaches infinity) of |1 / ((n + 1) / n)ⁿ|

= lim (as n approaches infinity) of |1 / (1 + (1 / n))ⁿ|

= 1 / e

Since L = 1 / e < 1, the series converges.

Example 2: Series with Trigonometric Functions

Consider the series:

Σ sin(n) from n=1 to infinity

To determine if this series diverges, we can use the Divergence Test:

lim (as n approaches infinity) of sin(n)

Since sin(n) oscillates between -1 and 1 and does not approach a specific limit, the limit does not exist, and certainly does not equal 0. Therefore, by the Divergence Test, the series diverges.

Example 3: Telescoping Series

A telescoping series is one where consecutive terms cancel each other out, leading to a simplified sum. While many telescoping series converge, some can diverge depending on the behavior of their terms.

Consider the series:

Σ (1 / n - 1 / (n + 1)) from n=1 to infinity

This series telescopes:

(1 - 1/2) + (1/2 - 1/3) + (1/3 - 1/4) + ...

The partial sums are:

S₁ = 1 - 1/2

S₂ = 1 - 1/3

S₃ = 1 - 1/4

Sₖ = 1 - 1/(k + 1)

lim (as k approaches infinity) of Sₖ = lim (as k approaches infinity) of (1 - 1/(k + 1)) = 1

Since the sequence of partial sums converges to 1, the series converges to 1.

However, if we consider a slightly different series:

Σ (1 / (n + 1) - 1 / n) from n=1 to infinity

This series also telescopes, but the terms are reversed:

(1/2 - 1) + (1/3 - 1/2) + (1/4 - 1/3) + ...

The partial sums are:

S₁ = 1/2 - 1 = -1/2

S₂ = 1/3 - 1 = -2/3

S₃ = 1/4 - 1 = -3/4

Sₖ = 1/(k + 1) - 1 = -k / (k + 1)

lim (as k approaches infinity) of Sₖ = lim (as k approaches infinity) of (-k / (k + 1)) = -1

Since the sequence of partial sums converges to -1, this series converges to -1.

Conclusion

Determining whether a series is divergent involves applying various tests and techniques, each suited to different types of series. The Divergence Test, Comparison Test, Limit Comparison Test, Integral Test, Ratio Test, and Root Test provide a comprehensive toolkit for analyzing series. Understanding these tests and their applications is crucial for students, engineers, and mathematicians alike. While some series clearly diverge based on simple criteria, others require more sophisticated analysis. The ability to discern divergence is fundamental to advanced mathematical analysis and has practical implications across various fields.