Chapter 3 – Sequence and String

CSNB 143 Discrete Mathematical

Structures

OBJECTIVESStudents should be able to differentiate few

characteristics of sequence. Students should be able to use sequence

and strings.Students should be able to concatenate

string and know how to use them.

What, which, where, whenKnowledge about sequenceFinite (Clear / Not Clear )Infinite (Clear / Not Clear )Recursive (Clear / Not Clear )Explicit (Clear / Not Clear )Increasing (Clear / Not Clear )Decreasing (Clear / Not Clear )

String (Clear / Not Clear )

Concatenation (Clear / Not Clear )

Subsequence (Clear / Not Clear )

SequenceA list of objects in its order. That is, taking

order as an important thing. A list in which the first one should be in front,

followed by the second element, third element and so on.

List might be ended after n, n N and it is named as Finite Sequence. We called n as an index for that sequence.

List might have no ending value, and this is called as Infinite Sequence.

Elements might be redundancy.

Ex 1: S = 2, 4, 6, …, 2n S = S1, S2, S3, … Sn

where S1=2, S2= 4, S3=6, … Sn = 2n

Ex 2:T = a, a, b, a, bwhere T1=a, T2=a, T3=b, T4=a, T5=b

If the sequence is depending on the previous value, it is called Recursive Sequence.

If the sequence is not depending on the previous value, in which the value can be directly retrieved, it is called Explicit Sequence.

Ex 3:An = An-1 + 5; A1 = 1, 2 n < , this is a recursive

sequence

where: A2 = A1 + 5A3 = A2 + 5

Ex 4:An = n2 + 1; 1 n < , this is an explicit sequencewhere: A1 = 1 + 1 = 2A2 = 4 + 1 = 5A3 = 9 + 1 = 10

That is, we can get the value directly, without any dependency to previous value.

Both recursive and explicit formula can have both finite and infinite sequence.

Ex 5: Consider all the sequences below, and identify which sequence is recursive/explicit and finite/infinite.

a)C1 = 5, Cn = 2Cn-1, 2 n 6b)D1 = 3, Dn = Dn-1 + 4c)Sn = (-4)n, 1 n d)Tn = 92 – 5n, 1 n 5

Both sequences also can have an Increasing or Decreasing sequence.

A sequence is said to be increased if for each Sn, the value is less than Sn + 1 for all n, Sn Sn + 1 ; all n

A sequence is said to be decreased if for each Sn the value is bigger than Sn + 1 for all n,Sn Sn + 1 ; all n

Ex 6: Determine either this sequence in increasing or decreasing.

Sn = 2(n + 1), n 1Xn = (½)n, n 1S = 3, 5, 5, 7, 8, 8, 13

StringSequences or letters or other symbols that is

written without commas are also referred as strings.

An infinite string such as abababa… may be regarded as infinite sequence of a,b,a,b,a,b,a…

The set corresponding to sequence is simply the set of all distinct elements in the sequence.E.g 1: 1,4,8,9,2… is {1,4,8,9,2…}E.g 2 : a,b,a,b,a,b,a… is simply {a, b}

A string over A set is a finite sequence of elements from A.

Let A = {a, b, c}. If we let A1 = b, A2 = a, A3 = a, A4 = c

Then we obtain a string over A. The string is written baac.

Since a string is a sequence, order is taken into account. For example the string baac is different from acab.

Repetition in a string can be specified by superscript. For example the string bbaaac may be written b2a3c.

The string with no element is call the null string and is denoted as . We let set A* denote the set of all strings over A, including the null string.

Ex 10: Let say A = {a, b, c, …, z} Then

A* = {aaaa, computer, denda, pqr, sysrq,… }Or let X = {a, b }. Some elements of X* are:

a, b, baba, , b2a29ba

That is, all finite sequence that can be build from A, contains all words either it has any meaning or not, regardless its length.

The number of elements in any string A is called Elements’ Length, denoted as |A|.

Ex 11:If A = abcde…z, then |A| = 26.

ConcatenationIf W1 and W2 are two strings, the string

consisting of W1 followed by W2 written W1. W2 is called concatenation of W1 and W2 :

W1.W2 =A1A2A3…AnB1B2B3…Bm

where W1.W2

And it is known that W1. = W1 and .W1 = W1

Ex 12: Let say R = aabc, S = dacbSo, R.S = aabcdacb S.R =

dacbaabc R. = aabc .R = aabc

SubsequenceIt is quite different from what we have learn in

subset A new sequence can be build from original

sequence, but the order of elements must remains.Ex 13: T = a, a, b, c, q

where T1=a, T2=a, T 3=b, T4=c, T5=q

S = b, c is a subsequence of Tbut R = c, b is not a subsequence of T

*Take note that the order in which b and c appears must be the same with the original sequence.

Exercise1.List all string on X = {0, 1}, with length 2.2.With your own words, explain the meaning

of sequence. What is the basic difference between sequence and set?