Unit - 5

Infinite Series

5.1 Infinite Series: Convergence of series

Sequence–

A function f : N , where S is a non-empty set , is called sequence , for each nϵN.

The sequence is written as f(1) , f(2) , f(3) , f(4)……….f(n).

Any sequence f(n) can be denoted as <f(n)> or {f(n)} or (f(n)).

Suppose f(n) =

Then it can be written as -    and can be denoted as <>or  {} or ()

is the n’th term of the sequence.

Example:  suppose we have a sequence – 1 , 4 , 9 , 16 ,……….. And its n’th term is

This sequence can be written as  -<>

Example: its n’th term will be .and can be written as <>

Types of sequences –

1. Finite sequence- A sequence which has finite number of terms is called finite sequence.

2. Infinite sequence-  A sequence which is not finite , called infinite sequence.

Limit of a Sequence- A sequence <> is said to tend to limit “l” , when given any positive number ‘ϵ’ , however small , we can always find a integer ‘m’ such that | – l| <ϵ , for every for all, n≥m , and we can define this as follows,

Example: If    , then the limit of will be,

  = =  = ½

Hence the limit of the sequence is 1/2.

Some important limits to remember-

Convergent sequence- A sequence Sn is said to be convergent when it tends to a finite limit. That means the limit of a sequence Sn will be always finite in case of convergent sequence.

Divergent sequence- when a sequence tends to ±∞ then it is called divergent sequence.

Oscillatory sequence- when a sequence neither converges nor diverges then it is called oscillatory sequence.

Note- a sequence which neither converges nor diverges , is called oscillatory sequence.

A sequence converges to zero Is called null.

Example-1:  consider a sequence 2, 3/2 , 4/3 , 5/4, …….. Here Sn =  1 + 1/n

Sol. As we can see that the sequence Sn is convergent and has limit 1.

According to def.

Example-2: consider a sequence Sn=  n² + (-1)ⁿ.

Sol. Here we can see that, the sequence Sn is divergent  as it has infinite limit.

Example: suppose , here the sequence is said to be oscillate. Because it is between -2 and 2.

Series

Infinite series- If  is a sequence, then is called the infinite series.

It is denoted by .

Examples of infinite series-

Convergent series - suppose n→∞ , Sn→ a finite limit  ‘s’  , then the seires Sn is said to be convergent .

We can denote it as,

Divergent series- when Sn tends to infinity then the series is said to be divergent.

Oscillatory series- when Sn does not tends to a unique limit (finite or infinite), then it is called Oscillatory series.

Properties of infinite series –

1. The convergence and divergence of an infinite series is unchanged addition of deletion of a finite number of term from it.

2. If positive terms of convergent series change their sign, then the series will be convergent.

3. Let   converges to s, let k be a non-zero fixed number  then converges to ks.

4. Let converges to ‘l’ and converges to ‘m’.

Example-1: check whether the series is convergent or divergent. Find its value in case of convergent.

Sol.  As we know that,

Sn = 

Therefore,

Sn = 

Now find out the limit of the sequence,

  = ∞

Here the value of the limit is infinity, so that the series is divergent as sequence diverges.

Example-2: check whether the series  is convergent or divergent. Find its value in case of convergent.

Sol. The general formula for this series is given by,

Sn =  = )

We get,

) = 3/2

Hence the series is convergent and its values is 3/2.

Example-3: check whether the series  is convergent or divergent.         

Sol. The general formula can be written as,

We get on applying limits,

) = 3/4

This is the convergent series and its value is 3 / 4

Example-4: check whether the following series is convergent or divergent. If convergent, find its value.

Sol. n’th term of the series will be,

General properties of series

The general properties of series are-

1. The nature of a series does not change by multiplication of all terms by a constant k.

2. The nature of a series does not change by adding or deleting of a finite number of terms.

3. If two series and are convergent, then is also convergent.

Exampple-1: prove that the following series is convergent and find its sum.

Sol. Here,

And

Hence the series is convergent and the limit is 1/2 .

Example-2: Test the convergence of the series-

Sol. Here we can see that the given series is in geometric progression

As its first term is 1 and common ratio is ½.

Then we know that the sum of n terms of a geometric progression is-

Hence the limit will be-

So that the series is convergent.

Key takeaways-

1. A function f: N , where S is a non-empty set, is called sequence, for each nϵN.
2. Any sequence f(n) can be denoted as <f(n)> or {f(n)} or (f(n)).
3. A sequence Sn is said to be convergent when it tends to a finite limit. That means the limit of a sequence Sn will be always finite in case of convergent sequence.
4. When a sequence tends to ±∞ then it is called divergent sequence.
5. A sequence which neither converges nor diverges, is called oscillatory sequence.
6. Suppose n→∞, Sn→ a finite limit ‘s’, then the series Sn is said to be convergent.
7. When Sn tends to infinity then the series is said to be divergent.
8. The convergence and divergence of an infinite series is unchanged addition or deletion of a finite number of terms from it.
9. If positive terms of convergent series change their sign, then the series will be convergent
10. Let converges to ‘l’ and converges to ‘m’.
11. The nature of a series does not change by multiplication of all terms by a constant k.
12. The nature of a series does not change by adding or deleting of a finite number of terms.
13. If two series and are convergent, then is also convergent.

5.2 Comparison test

Positive term series-

If all the terms in an infinite series are positive after few negative terms , then the series said to be a positive term series.

Suppose,

-22-65+ 55 +69 99+125+………….is a positive term series.

If we remove these negative terms, then the nature of the series does not change.

Comparison test-

Statement-

Suppose we have two series of positive terms and then,

, where k is a finite number, then both series converges or diverges together.

Proof- we know that by the definition of limits, there exist a positive number epsilon(ε)

Which is very small. Such that

According to definition (comparison test)

||<ε for n>m, that means

k-ε<    for n>m

Ignoring the first m terms of the series,

We get

k-ε<    for n>m  for all n  ………………..(1)

There will be two cases-

Case-1: is convergent, then

() = r (say) , where r is finite number

From (1),

()<() =

Therefore  is also convergent.

Case-2: is divergent, then

()→∞   …………………………..(2)

From eq. (1)

Then

()<() 

From(2)

()→∞  

Hence, is also divergent.

Example: Test the convergence of the following series.

Sol. We have  

First we will find  and the

And

Here, we can see that, the limit is finite and not zero,

Therefore, and converges or diverges together.

Since is of the form  where p = 2>1

So that, we can say that,

is convergent, so that will also be convergent.

Example: Test the convergence of the following series-

Sol. Here we have the series,

Now,

Now compare

We can see that the limit is finite and not zero.

Here  and converges or diverges together since,

is the form of here p = 1,

So that,

is divergent then is also divergent.

Example: Show that the following series is convergent.

Sol. 

Suppose,

Which is finite and not zero.

By comparison test and converge or diverge together.

But,

Is convergent. So that is also convergent.

Example: Test the series:

Sol. The series is,

Now,

Take,

Which is finite and not zero.

By comparison test and converge or diverge together.

But,

Is divergent. ( p = ½)

So that is divergent.

Key takeaways-

1. If is a series of positive terms such that then,

2. If k<1, the series will be convergent.

3. If k>1, then the series will be divergent.

4. Suppose we have two series of positive terms and then,

, where k is a finite number, then both series converges or diverges together.

5.3 p-Test

The infinite series,

Is convergent , when

(1) convergent when p >1

(2) divergent if p <= 1

Proof:

Let p>1

Similarly,

And so on.

Adding we get,

The right hand side of the above inequality is an infinite geometric series , the sum of the geometric series is finite hence the given series is convergent.

Example-1: Test the convergence of the following series:

Sol.

Here we take,

Which not zero and finite,

So by comparison test, and both converges or diverges, but by p-series test

Is convergent .so that  is convergent.

Example-2: Test the convergence of the given series:

Sol.  Here,

Take,

is divergent by p-series test. (p = 0 <1)

Example-3: Test the convergence of the series:

Sol.

By p-series test,

Is convergent ( p = 3>1).

5.4 Cauchy’s nth Root test

Let be a series of positive terms and let

Then is convergent when  l<1 and diverges when l >1.

Proof:  case-1:

Or

Since,

Is a geometric series with common ratio <1  so that the series will be convergent.

Case- 2:

By the limit concept, we can find a number,

That means

After 1st ‘r’ terms , each term is > 1

So that the series is divergent.

Example: Test the convergence of the series whose nth term is given below-

Sol.

By root test is convergent.

Example: Test the convergence of the series whose nth term is given below-

Sol.

By root test is convergent.

Example: show that the following series is convergent.

Sol.

By root test is convergent.

Example: Test the convergence of the following series:

Sol. Here, we have,

Therefore, the given series is convergent.

5.5 D Alembert’s Ratio Test

Statement- suppose is a series of positive terms  such that then,

1. If k<1, the series will be convergent.

2. If k>1, then the series will be divergent.

Proof:

Case-1:

We know that from the definition of limits, it follows,

But,

Is the finite quantity. So is convergent.

Case-2:

There could be some finite terms in starting which will never satisfy the condition,

In this case we can find a number ‘m’,

Ignoring the first ‘m’ terms, if we write the series as

We have , in this case

So that is divergent.

Example: Test the convergence of the series whose n’th term is given below-

n’th term =

Sol. We have

  and

By D’Alembert ratio test,

So that by D’Alembert ratio test, the series will be convergent.

Example: Test for the convergence of the n’th term of the series given below-

Sol. We have,

Now, by D’Alembert ratio test converges if and diverges if 

At x = 1, this test fails.

Now, when x = 1

The limit is finite and not zero.

Then by comparison test, converges or diverges together.

Since is the form of , in which

Hence diverges then will also diverge.

Therefore in the given series converges if x<1 and diverges if x≥1.

Key takeaways

1. If is a series of positive terms such that then,
2. If k<1, the series will be convergent.
3. If k>1, then the series will be divergent.

5.6 Rabies Test

Let be a series of positive terms and

Then,

(1) if k >1, is convergent   and

(2) if k < 1, then   is divergent

(3) this test fails if k = 1.

Proof: let us consider the series,

Case-1: in this case,

We choose a number ‘p’ for all k > p >1, comparing the series   with which is divergent,

We get will converge if after some fixed number of terms,

That means,

If k >p, which Is true . Hence is convergent.

We can prove the second case similarly.

Example-1: Test the convergence of the following series.

Sol. Neglecting the first term the series can be written as,

So that,

By ratio test  converges if |x|<1 and diverges if |x|>1, but if |x| = 1 the the test fails,

Then ,

By Rabies’s test  converges hence the given series is convergent when |x|≤ 1 and divergent If |x| >1.

Example-2: Test the convergence of the series,

Sol. As we will neglect the first term, we get

By ration test  is convergent when x<1 and divergent when x>1, when x= 1,

The ratio test fails, then

By Rabee’s test is convergent, hence the given series is convergent when x≤ 1 and divergent If x >1.

Example-3: Test the nature of the following series:

Sol.

By ration test  is convergent when (x/4)<1 and divergent when x>4, when x= 4,

The ratio test fails, then

By Rabee’s test is convergent, hence the given series is convergent when x<4 and divergent If x >=4.

5.7 Logarithmic test

Statement-  Suppose is series with positive terms such that

Then,

1. If k>1, then the series is convergent.

2. If k<1, then the series is divergent.

Proof: if k>1

Compare with , if k>p>1 then converges.

Taking log on both sides, we get

k>p which is true as k>p>1

Hence is convergent.

When p<1,

Similarly when p<1, then is divergent.

At p = 1, then this test fails.

Example: Test the convergence of the following series:

Sol.  We have the series,

Here, And

Which gives,

  , the series is convergent.

If   , the series is divergent.

.

Thus the series is divergent.

References:

1. D. Poole, “Linear Algebra: A Modern Introduction”, Brooks/Cole, 2005.
2. N.P. Bali And M. Goyal, “A Text Book Of Engineering Mathematics”, Laxmi Publications, 2008.
3. B.S. Grewal, “Higher Engineering Mathematics”, Khanna Publishers, 2010.
4. V. Krishnamurthy, V. P. Mainra And J. L. Arora, “An Introduction To Linear Algebra”, Affiliated East-West Press, 2005.
5. G.B. Thomas and R.L. Finney, “Calculus and Analytic Geometry”, Pearson, 2002.