Ratio test

Ratio test
In mathematics, the ratio test is a test (or "criterion") for the convergence of a series
where each term is a real or complex number and is nonzero when is large. The test was first published by Jean le Rond d'Alembert and is sometimes known as d'Alembert's ratio test or as the Cauchy ratio test.
The test
The usual form of the test makes use of the limit
The ratio test states that:
if L < 1 then the series converges absolutely;
if L > 1 then the series diverges;
if L = 1 or the limit fails to exist, then the test is inconclusive, because there exist both convergent and divergent series that satisfy this case.
It is possible to make the ratio test applicable to certain cases where the limit L fails to exist, if limit superior and limit inferior are used. The test criteria can also be refined so that the test is sometimes conclusive even when L = 1. More specifically, let
.
Then the ratio test states that:
if R < 1, the series converges absolutely;
if r > 1, the series diverges; or equivalently if for all large n (regardless of the value of r), the series also diverges; this is because is nonzero and increasing and hence does not approach zero;
the test is otherwise inconclusive.
If the limit L in exists, we must have L = R = r. So the original ratio test is a weaker version of the refined one.
Examples
Convergent because L < 1
Consider the series
Applying the ratio test, one computes the limit
Since this limit is less than 1, the series converges.
Divergent because L > 1
Consider the series
Putting this into the ratio test:
Thus the series diverges.
Inconclusive because L = 1
Consider the three series
The first series (1 + 1 + 1 + 1 + ⋯) diverges, the second one (the one central to the Basel problem) converges absolutely and the third one (the alternating harmonic series) converges conditionally. However, the term-by-term magnitude ratios of the three series are respectively and . So, in all three cases, one has that the limit is equal to 1. This illustrates that when L = 1, the series may converge or diverge, and hence the original ratio test is inconclusive. In such cases, more refined tests are required to determine convergence or divergence.
Proof
Below is a proof of the validity of the original ratio test.
Suppose that . We can then show that the series converges absolutely by showing that its terms will eventually become less than those of a certain convergent geometric series. To do this, consider a real number r such that . This implies that for sufficiently large n; say, for all n greater than N. Hence for each n > N and i > 0, and so
That is, the series converges absolutely.
On the other hand, if L > 1, then for sufficiently large n, so that the limit of the summands is non-zero. Hence the series diverges.
Extensions for L = 1
As seen in the previous example, the ratio test may be inconclusive when the limit of the ratio is 1. Extensions to the ratio test, however, sometimes allows one to deal with this case.
In all the tests below one assumes that Σan is a sum with positive an. These tests also may be applied to any series with a finite number of negative terms. Any such series may be written as:
where aN is the highest-indexed negative term. The first expression on the right is a partial sum which will be finite, and so the convergence of the entire series will be determined by the convergence properties of the second expression on the right, which may be re-indexed to form a series of all positive terms beginning at n=1.
Each test defines a test parameter (ρn) which specifies the behavior of that parameter needed to establish convergence or divergence. For each test, a weaker form of the test exists which will instead place restrictions upon limn->∞ρn.
All of the tests have regions in which they fail to describe the convergence properties of Σan. In fact, no convergence test can fully describe the convergence properties of the series. This is because if Σan is convergent, a second convergent series Σbn can be found which converges more slowly: i.e., it has the property that limn->∞ (bn/an) = ∞. Furthermore, if Σan is divergent, a second divergent series Σbn can be found which diverges more slowly: i.e., it has the property that limn->∞ (bn/an) = 0. Convergence tests essentially use the comparison test on some particular family of an, and fail for sequences which converge or diverge more slowly.
De Morgan hierarchy
Augustus De Morgan proposed a hierarchy of ratio-type tests
The ratio test parameters below all generally involve terms of the form . This term may be multiplied by to yield . This term can replace the former term in the definition of the test parameters and the conclusions drawn will remain the same. Accordingly, there will be no distinction drawn between references which use one or the other form of the test parameter.
1. d'Alembert's ratio test
The first test in the De Morgan hierarchy is the ratio test as described above.
2. Raabe's test
This extension is due to Joseph Ludwig Raabe. Define:
(and some extra terms, see Ali, Blackburn, Feld, Duris (none), Duris2)
The series will:
Converge when there exists a c>1 such that for all n>N.
Diverge when for all n>N.
Otherwise, the test is inconclusive.
For the limit version, the series will:
Converge if (this includes the case ρ = ∞)
Diverge if .
If ρ = 1, the test is inconclusive.
When the above limit does not exist, it may be possible to use limits superior and inferior. The series will:
Converge if
Diverge if
Otherwise, the test is inconclusive.
Proof of Raabe's test
Defining , we need not assume the limit exists; if , then diverges, while if the sum converges.
The proof proceeds essentially by comparison with . Suppose first that . Of course
if then for large , so the sum diverges; assume then that . There exists such that for all , which is to say that . Thus , which implies that
for ; since this shows that diverges.
The proof of the other half is entirely analogous, with most of the inequalities simply reversed. We need a preliminary inequality to use
in place of the simple that was used above: Fix and . Note that
. So ; hence .
Suppose now that . Arguing as in the first paragraph, using the inequality established in the previous paragraph, we see that there exists such that for ; since this shows that converges.
3. Bertrand's test
This extension is due to Joseph Bertrand and Augustus De Morgan.
Defining:
Bertrand's test asserts that the series will:
Converge when there exists a c>1 such that for all n>N.
Diverge when for all n>N.
Otherwise, the test is inconclusive.
For the limit version, the series will:
Converge if (this includes the case ρ = ∞)
Diverge if .
If ρ = 1, the test is inconclusive.
When the above limit does not exist, it may be possible to use limits superior and inferior. The series will:
Converge if
Diverge if
Otherwise, the test is inconclusive.
4. Extended Bertrand's test
This extension probably appeared at the first time by Margaret Martin in 1941. A short proof based on Kummer's test and without technical assumptions (such as existence of the limits, for example) was provided by Vyacheslav Abramov in 2019.
Let be an integer, and let denote the th iterate of natural logarithm, i.e. and for any ,
.
Suppose that the ratio , when is large, can be presented in the form
(The empty sum is assumed to be 0. With , the test reduces to Bertrand's test.)
The value can be presented explicitly in the form
Extended Bertrand's test asserts that the series
Converge when there exists a such that for all .
Diverge when for all .
Otherwise, the test is inconclusive.
For the limit version, the series
Converge if (this includes the case )
Diverge if .
If , the test is inconclusive.
When the above limit does not exist, it may be possible to use limits superior and inferior. The series
Converge if
Diverge if
Otherwise, the test is inconclusive.
For applications of Extended Bertrand's test see birth–death process.
5. Gauss's test
This extension is due to Carl Friedrich Gauss.
Assuming an > 0 and r > 1, if a bounded sequence Cn can be found such that for all n:
then the series will:
Converge if
Diverge if
6. Kummer's test
This extension is due to Ernst Kummer.
Let ζn be an auxiliary sequence of positive constants. Define
Kummer's test states that the series will:
Converge if there exists a such that for all n>N. (Note this is not the same as saying )
Diverge if for all n>N and diverges.
For the limit version, the series will:
Converge if (this includes the case ρ = ∞)
Diverge if and diverges.
Otherwise the test is inconclusive
When the above limit does not exist, it may be possible to use limits superior and inferior. The series will
Converge if
Diverge if and diverges.
Special cases
All of the tests in De Morgan's hierarchy except Gauss's test can easily be seen as special cases of Kummer's test:
For the ratio test, let ζn=1. Then:
For Raabe's test, let ζn=n. Then:
For Bertrand's test, let ζn=n ln(n). Then:
Using and approximating for large n, which is negligible compared to the other terms, may be written:
For Extended Bertrand's test, let From the Taylor series expansion for large we arrive at the approximation
where the empty product is assumed to be 1. Then,
Hence,
Note that for these four tests, the higher they are in the De Morgan hierarchy, the more slowly the series diverges.
Proof of Kummer's test
If then fix a positive number . There exists
a natural number such that for every
Since , for every
In particular for all which means that starting from the index
the sequence is monotonically decreasing and
positive which in particular implies that it is bounded below by 0. Therefore, the limit
exists.
This implies that the positive telescoping series
is convergent,
and since for all
by the direct comparison test for positive series, the series
is convergent.
On the other hand, if , then there is an N such that is increasing for . In particular, there exists an for which for all , and so diverges by comparison with .
Tong's modification of Kummer's test
A new version of Kummer's test was established by Tong. See also
for further discussions and new proofs. The provided modification of Kummer's theorem characterizes
all positive series, and the convergence or divergence can be formulated in the form of two necessary and sufficient conditions, one for convergence and another for divergence.
Series converges if and only if there exists a positive sequence such that
Series diverges if and only if there exists a positive sequence such that and
The first of these statements can be simplified as follows:
Series converges if and only if there exists a positive sequence such that
The second statement can be simplified similarly:
Series diverges if and only if there exists a positive sequence such that and
However, it becomes useless, since the condition in this case reduces to the original claim
Frink's ratio test
Another ratio test that can be set in the framework of Kummer's theorem was presented by Orrin Frink 1948.
Suppose is a sequence in ,
If , then the series converges absolutely.
If there is such that for all , then diverges.
This result reduces to a comparison of with a power series , and can be seen to be related to Raabe's test.
Ali's second ratio test
A more refined ratio test is the second ratio test:
For define:
By the second ratio test, the series will:
Converge if
Diverge if
If then the test is inconclusive.
If the above limits do not exist, it may be possible to use the limits superior and inferior. Define:
Then the series will:
Converge if
Diverge if
If then the test is inconclusive.
Ali's mth ratio test
This test is a direct extension of the second ratio test. For and positive define:
By the th ratio test, the series will:
Converge if
Diverge if
If then the test is inconclusive.
If the above limits do not exist, it may be possible to use the limits superior and inferior. For define:
Then the series will:
Converge if
Diverge if
If , then the test is inconclusive.
Ali--Deutsche Cohen φ-ratio test
This test is an extension of the th ratio test.
Assume that the sequence is a positive decreasing sequence.
Let be such that exists. Denote , and assume .
Assume also that
Then the series will:
Converge if
Diverge if
If , then the test is inconclusive.
See also
Root test
Radius of convergence
Footnotes
References
.
: §8.14.
: §3.3, 5.4.
: §3.34.
: §2.36, 2.37.
Convergence tests
Articles containing proofs
it:Criteri di convergenza#Criterio del rapporto (o di d'Alembert)