# Study Notes on Real Sequences (Definitions and Examples)

In this study note, I have discussed and explained about real sequences — a very important topic in Real Analysis and Calculus.

I will cover the topic, definitions and examples under the following headings:

**Table of Contents**

## Real Sequence: A sequence of real numbers

A sequence of real numbers (or a real sequence) is defined as a function $ f: \mathbb{N} \to \mathbb{R}$ , where $ \mathbb{N}$ is the set of natural numbers and $ \mathbb{R}$ is the set of real numbers.

Thus, $ f(n)=r_n, \ n \in \mathbb{N}, \ r_n \in \mathbb{R}$ is a function which produces a sequence of real numbers $ r_n$ . It's customary to write a sequence as form of functions in brackets, e.g.; $ \langle f(n) \rangle$ , $ { f(n) }$ . We can alternatively represent a sequence as the function with natural numbers as subscripts, e.g., $ \langle f_n \rangle$ , $ { f_n }$ .

This alternate method is a better representation of a sequence as it distinguishes 'a sequence' from 'a function'. We shall use $ \langle f_n \rangle$ notation and when writen $ \langle f_n \rangle$ , we mean $ \langle f_1, f_2, f_3, \ldots, f_n, \ldots \rangle$ a sequence with infinitely many terms.

Since all of $ { f_1, f_2, f_3, \ldots, f_n, \ldots }$ are real numbers, this kind of sequence is called a sequence of real numbers or real sequences.

### Examples of Real Sequences

- If $ f(x)=\dfrac{1}{x} \forall x \in \mathbb{R}$ is a real-valued-function, $ f(n)=\dfrac{1}{n} \forall n \in \mathbb{N}$ is a real sequence.
- Putting consecutive values of $ n \in \mathbb{N}$ in $ f(n)=\dfrac{1}{n}$ we obtain a real-sequence
- for
*n=1; f(1)=1* - for
*n=2; f(2)=1/2* - for
*n=3; f(3)=1/3* - …
*…* - for
*n=n; f(n)=1/n* - …
*...*

- for
- This real sequence can be represented by
- $ \langle \dfrac{1}{n} \rangle := \langle 1, \dfrac{1}{2}, \dfrac{1}{3}, \ldots, \dfrac{1}{n}, \ldots$ .

- Putting consecutive values of $ n \in \mathbb{N}$ in $ f(n)=\dfrac{1}{n}$ we obtain a real-sequence
- $ \langle {(-1)}^n \rangle$ is the sequence $ \langle -1, 1, -1, 1, \ldots, {(-1)}^n, \ldots \rangle$ .
- $ \langle -3n \rangle$ is the sequence $ \langle -3, -6, -9, \ldots, -3n, \ldots \rangle$
- A sequence can also be formed by a recurrence relation with boundary values. If $ f_n= f_{n-1}+f_{n-2} \ \text{for} n \ge 2$ and $ f_0=f_1=1$ , then we obtain the sequence $ \langle f_n \rangle$ as
- for
*n=1 ;*$ f_1=1$ (given) - for
*n=2;*$ f_2=f_1 +f_0=1+1=2$ (given $ f_0=1=f_1$ ) - for
*n=3;*$ f_3=f_2+f_1=2+1=3$ - for
*n=4;*$ f_4=f_3+f_2=3+2=5$ - and so on...
- This sequence, $ \langle 1, 1,2, 3, 5, 8, 13, 21, \ldots \rangle$ is a real-sequence known as
*Fibonacci Sequence.*

- for

### Range Set of a Sequence

The set of all *'distinct' elements *of a sequence is called the range set of the given sequence.

For example:

- The range set of $ \langle \dfrac{1}{n}\rangle:= \{ \dfrac{1}{n} : n \in \mathbb{N} \}$ , which is an infinite set.
- The range set of $ \langle {(-1)}^n \rangle := \{ -1, 1 \}$ , a finite set.

** Remark: **The range set of a sequence may be either infinite or finite, but a sequence always has an infinite number of elements.

### Sub-sequence of the Sequence

A sub-sequence of the sequence is *another sequence *containing *some *of the values of the sequence in the same order as in the original sequence. Alternatively, a sub-sequence of the sequence is another sequence whose range set is a subset of the range set of the sequence.

For example:

*<1, 3, 5, 7, …>*is a sub-sequence of the sequence*<1, 2, 3, 4, ...>.**<1, 5, 13, 21, ...>*is a sub-sequence of the sequence*<1,1,2,3,5,8,13,21, 34, ...>.**<1,1,1,1,1,...>*is a sub-sequence of the sequence*<-1, 1, -1, 1, ...>.*Since, the sequence*<1,1,1,1,...>*has only one value for each term, it's called a**constant sequence****.**

** Remark:** A sub-sequence is also a sequence; hence it satisfies and follows all the properties of a sequence.

### Equality of two sequences

Two sequences $ \langle S_n \rangle$ and $ \langle T_n \rangle$ are said to be equal, if and only if $ S_n=T_n, \forall n \in \mathbb{N}$ .

For example: The sequences $ \langle \dfrac{n+1}{n} \rangle$ and $ \langle 1+\dfrac{1}{n} \rangle$ are equal to each other.

**Remark:**** **From the *definition* the sequences *<-1,1,-1,1, ...> *and *<1,-1,1,-1,...> *are not equal to each other, though they look alike and has same range set.

### Algebra of Sequences

Let $ \langle S_n \rangle$ and $ \langle T_n \rangle$ be two sequence, then the sequences having *n*-th terms $ S_n+T_n, \ S_n-T_n, \ S_n \cdot T_n, \ \dfrac{S_n}{T_n}$ (respectively) are called the SUM, DIFFERENCE, PRODUCT, QUOTIENT of $ \langle S_n \rangle$ and $ \langle T_n \rangle$ .

*For example: *The sequence <1, 8, 19,30, ...> is the sum of sequences <0, 1, 2, 3, ...> and <1, 7, 17, 27, ...> obtained after adding n-th term of one sequence to corresponding n-th term of other sequence. Similarly, other operations can be carried.

If $ S_n \ne 0 \forall n$ , then the sequence $ \langle \dfrac{1}{S_n} \rangle$ is known as the *reciprocal *of the sequence $ \langle S_n \rangle$ .

*For example: *$ \langle \dfrac{1}{1}, \dfrac{-1}{2}, \dfrac{1}{3}, \ldots \rangle$ is the reciprocal of the sequence $ \langle 1, -2, 3, \ldots \rangle$ .

* Remark:* The sequences

*<-1,1,-1,1, ...>*and

*<1,-1,1,-1,...>*have their reciprocals equal to the original sequence, hence these are called

*identity-sequences.*

If $ c \in \mathbb{R}$ then the sequence with *n*-th term $ cS_n$ is called the scalar multiple of sequence $ \langle S_n \rangle$ . This sequence is denoted by $ \langle cS_n \rangle$ .

### Bounds of a Sequence

- A sequence $ \langle S_n \rangle$ is said to be
, if there exists a real number**bounded above***M*such that $ S_n \le M, \forall n \in \mathbb{N}$ .*M*is called an upper bound of the sequence $ \langle S_n \rangle$ . - A sequence $ \langle S_n \rangle$ is said to be
, if there exists a real number**bounded below***m*such that $ S_n \ge m, \forall n \in \mathbb{N}$ .*m*is called a lower bound of the sequence $ \langle S_n \rangle$ . - A sequence $ \langle S_n \rangle$ is said to be
**bounded***,*if it is both bounded above and bounded below. Thus, if $ \langle S_n \rangle$ is a bounded sequence, there exist two real numbers*m & M*such that $ m \le S_n \le M \forall n \in \mathbb{N}$ . - The least real number
*M,*if exists, of the set of all upper bounds of $ \langle S_n \rangle$ is called the least upper bound (supremum) of the sequence $ \langle S_n \rangle$ . - The greatest real number
*m*, if exits, of the set of all lower bounds of $ \langle S_n \rangle$ is called the greatest lower bound (infimum) of the sequence $ \langle S_n \rangle$ .

* Remark:* If the range set of a sequence is finite, then the sequence is always bounded.

Examples:

- The sequence $ \langle n^3 \rangle := \langle 1, 8, 27, \ldots \rangle$ is bounded below by 1, but is not bounded above.
- The sequence $ \langle \dfrac{1}{n} \rangle := \langle 1, \dfrac{1}{2}, \dfrac{1}{3}, \ldots$ is bounded as it has the range set (0, 1], which is finite.
- The sequence $ \langle {(-1)}^n \rangle := \langle -1, 1, -1, \ldots$ is also bounded.

## Convergent Sequence

A sequence $ \langle S_n \rangle$ is said to converge to a real number *l* if for each $ \epsilon$ >0, there exists a positive integer *m* depending on $ \epsilon$ , such that $ |S_n-l|$ < $ \epsilon \ \forall n \ge m$ .

This number *l* is called the limit of the sequence $ \langle S_n \rangle$ and we write this fact as $ \lim_{n \to \infty} S_n=l$ and the sequence itself is called a convergent sequence. From now on, we'll use $ \lim S_n=l$ to represent $ \lim_{n \to \infty} S_n=l$ , unless stated.

### Important Theorems on Convergent Sequences and Limit

- (
*Uniqueness Theorem*) Every convergent sequence has a unique limit. - For a sequence $ \langle S_n \rangle$ of non-negative numbers, $ \lim S_n \ge 0$ .
- Every convergent sequence is bounded, but the converse is not necessarily true.
- Let $ \lim S_n= l$ and $ T_n=l'$ , then $ \lim (S_n +T_n) = l+l'$ , $ \lim (S_n -T_n) = l-l'$ and $ \lim S_n \cdot T_n = l \cdot l'$ .
- Let $ \langle S_n \rangle$ and $ \langle T_n \rangle$ be two sequences such that $ S_n \le T_n$ , then $ \lim S_n \le \lim T_n$ .
- If $ \langle S_n \rangle$ converges to
*l*, then $ \langle |S_n| \rangle$ converges to |*l*|. In other words, if $ \lim S_n = l$ then $ \lim |S_n| =|l|$ . - (
*Sandwitch Theorem*) If $ \langle S_n \rangle$ , $ \langle T_n \rangle$ and $ \langle U_n \rangle$ be three sequences such that- $ S_n \le T_n \le U_n, \ \forall n \in \mathbb{N}$
- $ \lim S_n=l= \lim U_n$ ,

then $ \lim T_n=l$ .

- (
*Cauchy's first theorem on Limits*) If $ \lim S_n =l$ , then $ \dfrac{1}{n} \{ S_1+S_2+ \ldots +S_n \} =l$ . - (
*Cauchy's Second Theorem on Limits*) If $ \langle S_n \rangle$ is sequence such that $ S_n$ > $ 0, \ \forall n$ and $ \lim S_n =l$ , then $ \lim {(S_1 \cdot S_2 \cdot \ldots S_n)}^{1/n}= l$ . - Suppose $ \langle S_n \rangle$ is a sequence of positive real numbers such that $ \lim \dfrac{S_{n+1}}{S-n} =l$ , (
*l>0*), then $ \lim {(S_n)}^{1/n}=l$ . - (
*Cesaro's theorem*) If $ \lim S_n=l$ and $ \lim T_n=l'$ , then $ \lim \dfrac{1}{n} \{ S_1 T_1 + S_2 T_2 + \ldots + S_n t_n \} = l \cdot l'$

### Theorem on Sub-Sequences

- If a sequence $ \langle S_n \rangle$ converges to
*l*, then every subsequence of $ \langle S_n \rangle$ converges to*l*, i.e., every sub-sequence of a given sequence converges to the same limit.

## Divergent Sequence

A sequence $ \langle S_n \rangle$ is said to diverge if $ \lim_{n \to \infty} S_n = +\infty$ or $ \lim_{n \to \infty} S_n = -\infty$ .

## Oscillatory Sequence

- A sequence $ \langle S_n \rangle$ is said to oscillate finitely if

I. It's bounded.

II. It neither converges nor diverges. - A sequence $ \langle S_n \rangle$ is said to oscillate infinitely, if

I. It'sbounded.**not**

II. It neither converges nor diverges.

A sequence is said to be *non-convergent *if it's either divergent or oscillatory.

## Limit Points of a Sequence

A real number *P* is said to be a *limit point of a sequence* if every neighborhood of *P *contains an infinite number of elements of the given sequence. In other words, a real number *P* is a limit point of a sequence $ \langle S_n \rangle$ , if for a given $ \epsilon$ >0, $ S_n \in (P-\epsilon, P+\epsilon )$ for infinitely many values of *n*.

**Bolzano Weierstrass Theorem**

*Every bounded real sequence has a limit point. (Proof)*

**Remarks: **

- An unbounded sequence may or may not have a limit point.
- The greatest limit point of the bounded sequence $ \langle S_n \rangle$ is called the
*limit superior*of $ \langle S_n \rangle$ and is denoted by $ \lim \text{Sup} S_n$ . - The smallest limit point of the bounded sequence $ \langle S_n \rangle$ is called the
*limit inferior*of $ \langle S_n \rangle$ and is denoted by $ \lim \text{Inf} S_n$ . - limSup $ \ge$ limInf.

## Monotonic Sequences

A sequence $ \langle S_n \rangle$ is said to be monotonic if

either (i) $ S_{n+1} \ge S_n, \forall n \in \mathbb{N}$

or, (ii) $ S_{n+1} \le S_n, \forall n \in \mathbb{N}$ .

In the first case, the sequence is said to be* monotonically increasing* while in the second case, it's *monotonically decreasing*.

### Important Theorems on Monotonic Sequences

- A monotonically increasing sequence, which is bounded above, is convergent. (Otherwise, it diverges to $ +\infty$ .) It converges to its supremum.
- A monotonically decreasing sequence, which is bounded below, is convergent. (Otherwise, it diverges to $ -\infty$ .) It converges to its infimum.
- A monotonic sequence is convergent
*iff*it's bounded. (<==*combination of first two theorems*).

## Cauchy Sequences

A sequence $ \langle S_n \rangle$ is said to be a Cauchy's sequence if for every $ \epsilon$ >0, there exists a positive integer *m* such that $ |S_n -S_m|$ < $ \epsilon$ , whenever $ n \ge m$ .

### Important Properties of Cauchy Sequences

- Every Cauchy sequence is bounded. (proof)
- (
*Cauchy's general principle of convergence*) A sequence of real numbers converges if and only if it is a Cauchy sequence. (proof)

$ \Box$