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Convergence Test

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Divergence Test

Convergence Test

Also known as the n-th term test should be the first test to consider before using more advanced techniques. Unfortunately, this test cannot determine convergence.

If \(\underset{n\to\infty}{\lim}a_n\neq0\) then \( \sum a_n \) is guaranteed to diverge

If \(\underset{n\to\infty}{\lim}a_n=0\) then the test is inconclusive use another test.
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Geometric Series

Convergence Test

Usefull for computing sums whenever \(n\) always appears as an exponent. May require using properties of exponents to compute a sum.

\(\displaystyle\sum_{n=0}^{\infty}x^n=\frac{1}{1-x}\, \text{if}\, |x|\)<\(1\)

Otherwise if \(|x|\geq1\) then the series in question will diverge.
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Telescopic Series

Convergence Test

The terms in these series can be recalibrated as \(b_n=a_n-a_{n+1}\).

To obtain this format, you may be required to manipulated the terms algebraically by using techniques such as partial fractions or properties of logarithms.

\( \sum_{n=0}^{\infty}a_n-a_{n+1}=a_0-\underset{n\to\infty}{\lim}a_n\)
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Limit Comparison Test (LCT)

Convergence Test

Useful for analyzing rational functions. Let \(a_n,b_n\) be nonnegative sequences.

Compute \(\underset{n\to\infty}{\lim}\frac{a_n}{b_n}=L\)

If \(0\)<\(L\)<\(\infty\) then

\(\textstyle\sum a_n\) and \(\textstyle\sum b_n\) will mimic the same behavior, i.e. they both converge or they both diverge.

If \(L=0\) or \(L=\infty\) then it is recommended to consider using another test
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Direct Comparison Test (DCT)

Convergence Test

Let \(a_n,b_n,c_n\) be a nonnegtive sequence such that \(c_n\)<\(a_n\)<\(b_n\) then \(\sum c_n\)<\(\sum a_n\)<\(\sum b_n\)
  • If \(\sum c_n\) diverges then so does \(\sum a_n\)
  • If \(\sum b_n\) converges then so does \(\sum a_n\)
  • If \(\sum b_n\) divergess no information is provided
  • If \(\sum c_n\) converges no information is provided
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Integral Test

Convergence Test

Useful when transcendental functions are present such as \(\texttt{ln}(x)\). To apply this test you must first verify that the terms are positive and decreasing.

\(f(x)\) is a continuous extension of \(a_n\)

The improper integral \( \int_{k}^{\infty}f(x) dx \) and the infinite series \( \sum_{n=k}^{\infty}a_n\) will mimic the same behavior, i.e., they both converge or they both diverge.
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p-Series

Convergence Test

These series are helpful for comparing the convergence of a given series, especially when using the LCT or the DCT.

\( \displaystyle\sum_{n=1}^{\infty}\frac{1}{n^p} =\small{\left\{\begin{array}{ll} \text{Converges}\\ p>1\\[8pt] \text{Diverges}\\ p\leq1 \end{array} \right.} \)
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ln-series

Convergence Test

As with the p-series, these series help us to compare the convergence of a given series, especially when using the LCT or the DCT.

\( \displaystyle\sum_{n=2}^{\infty}\frac{1}{n\big(ln(n)\big)^p} =\small{\left\{\begin{array}{l} \text{Converges}\\ p>1\\[8pt] \text{Diverges}\\ p\leq1 \end{array} \right.} \)
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Generalized Geometric Series

Convergence Test

Usefull for computing sums whenever \(n\) always appears as an exponent. May require using properties of exponents to compute a sum.

\(\displaystyle\sum_{n=k}^{\infty}x^n=\frac{x^k}{1-x}\, \text{if}\, |x|\)<\(1\)

Otherwise if \(|x|\geq1\) then the series in question will diverge.
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Ratio Test

Convergence Test

The ratio test is an excellent test when exponentials or factorials are present. However, the Ratio Test always inclusive when analyzing a rational function.

\(\underset{n\to\infty}{\lim}\left|\frac{a_{n+1}}{a_n}\right|=L\)

  • \(L\)<\(1\) Series converges
  • \(L>\) Series diverges
  • \(L=1\) Test is inconclusive
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Root Test

Convergence Test

The root test is an excellent choice when powers, expoentials, and factorials are present. This test has more 'kick' than the Ratio Test.

\(\underset{n\to\infty}{\lim}\sqrt[n]{|a_n|}=L\)

  • \(L\)<\(1\) Series converges
  • \(L>1\) Series diverges
  • \(L=1\) Test is inconclusive
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k-Tier Telescopic Series

Convergence Test

A k-tier telescopic series is generalizationof the classical telescopic \(b_n=a_n-a_{n+1}\).

\( \displaystyle\sum_{n=0}^{\infty}a_n-a_{n+2}=\\ \quad a_0+a_1-2\underset{n\to\infty}{\lim}a_n\)

\(\displaystyle \sum_{n=0}^{\infty}a_n-a_{n+k}=\\ \quad a_0+\cdots+a_{k-1}-k\underset{n\to\infty}{\lim}a_n\)
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Alternating Series Test

Convergence Test

Useful for analyzing alternating series \(\textstyle\sum(-1)^na_n\), \(a_n\geq0\).

CAUTION: This test does not show divergece

Begin by showing \(\underset{n\to\infty}{\lim}a_n=0\). Then, prove \(a_n\) is a decreasing sequence by either applying the definition directly or using the first derivative test.

[Note, checking a few examples NEVER proves a statement]
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Alternating Series Test 2.0

Convergence Test

Another approach for determining the convergence of \(\sum(-1)^na_n\) involves using assessing the convergence of \(\sum |a_n|\)

If \(\sum|a_n|\) converges then the alternating series \(\sum(-1)^na_n\) will converge absolutely.
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Conditional vs Absolute Convergence

Convergence Test

To classify the convergence of an alternating series as either absolutely convergent or conditional congerneget we note the following definitions.

If \(\sum(-1)^na_n\) and \(\sum a_n\) both converge then the alternating series converges absolutely,

If \(\sum(-1)^na_n\) converges but \(\sum a_n\) diverges then the alternating series converges conditionally.

Alternating Series Error Term

Convergence Flowchart