Fibonacci Sequencebaruah/TEACHING/2018-2Fa… · Fibonacci Sequence Definition: ... Finding a...

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© 2019 McGraw-Hill Education Fibonacci Sequence Definition: The Fibonacci sequence, F 0 ,F 1 ,F 2 ,…: Initial Conditions: F 0 =0, F 1 = 1 Recurrence Relation: F n = F n−1 + F n−2 (May sometimes see different ones)

Transcript of Fibonacci Sequencebaruah/TEACHING/2018-2Fa… · Fibonacci Sequence Definition: ... Finding a...

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Fibonacci SequenceDefinition: The Fibonacci sequence, F0 ,F1 ,F2,…:• Initial Conditions: F0 = 0, F1 = 1• Recurrence Relation: Fn = Fn−1 + Fn−2

(May sometimes see different ones)

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Solving Recurrence RelationsFinding a formula for the nth term of the sequence generated by a recurrence relation is called solving the recurrence relation.

Such a formula is called a closed formula.

We will discuss methods for solving recurrence relations later

Fn =

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Summations 1

{ }

1

Sum of the terms , 1,...,

from the sequence

The notation:

represents

m m n

n

nn

j j jj m m j nj m

m m n

a a aa

a a a

a a a

= £ £=

+

+

+ + × × ×+

å å å

The variable j is called the index of summation. It runs through all the integers starting with its lower limit m and ending with its upper limit n.

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Summations 2

{ }

0 1 2 3

0

1

2 5 7 10

More generally for a set :

Examples:

1 1 1 11

2 3 4If 2,5,7,10 then

jj s

nn j

jj s

Sa

r r r r r r

iS a a a a a

Î

¥

Î

+ + + + × × ×+ =

+ + + + × × × =

= = + + +

å

å

å

å

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Product Notation

{ }

1

Product of the terms , 1,...,

from the sequence

The notation:

represents

m m n

n

nn

j j jj m m j nj m

m m n

a a aa

a a a

a a a

= £ £=

+

+

´ ´×××´

Õ Õ Õ

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Some Summation Examples

First n positive integers:

Arithmetic progression: a, a+d, a+2d, a+3d, …, a + nd

Geometric progression: a, ar, ar2, ar3, …, arn

- When r = 1 …

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Some Summation Examples

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Some Useful Summation FormulaeTABLE 2 Some Useful Summation Formulae.

Sum Closed From

( )0

0n

k

kar r

=

¹å1

, 11

nar a rr

+ -¹

-

1

n

kk

=å ( )1

2n n +

2

1

n

kk

=å ( )( )1 2 1

6n n n+ +

3

1

n

kk

=å ( )22 1

4n n +

0, 1k

k

=å | x |< 1

1 x-

1

0, 1k

k

kx¥

-

=å | x |<

( )21

1 x-

Geometric Series

Easily proved by induction

(Proved using calculus)

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Cardinality of Infinite Sets

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Cardinality

Definition: The cardinality of a set A is equal to the

cardinality of a set B, denoted |A| = |B|, if and only if

there is a one-to-one correspondence (i.e., a bijection)

from A to B.

If there is a one-to-one function (i.e., an injection) from

A to B, the cardinality of A is less than or the same as the

cardinality of B and we write |A| ≤ |B|.

When |A| ≤ |B| and A and B have different cardinality,

we say that the cardinality of A is less than the

cardinality of B and write |A| < |B|.

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Cardinality

Definition: A set that is either finite or has the same cardinality as the set of positive integers (Z+) is called countable.

A set that is not countable is uncountable.

When an infinite set is countable (countably infinite) its cardinality is ℵ0 (where ℵ is aleph, the 1st letter of the Hebrew alphabet). We write |S| = ℵ0 and say that S has cardinality “aleph null.”

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Showing that a Set is Countable

Definition: A set that is either finite or has the same cardinality as the set of positive integers (Z+) is called countable.

An infinite set is countable if and only if there is a one-to-one correspondence between it and the set of positive integers (Z+).

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Showing that a Set is CountableExample 1: Show that the set of positive even integers E is countable set.Solution: Let ( ) 2 .

1 2 3 4 5 6

2 4 6 8 10 12

f x x=

Then f is a bijection from Z+ to ETo show that it is one-to-one, suppose that f(n) = f(m). Then 2n = 2m, and so n = m. To see that it is onto, suppose that t is an even positive integer. Then t = 2k for some positive integer k and f(k) = t.

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Showing that a Set is Countable

Example 2: Show that the set of integers Z is countable.

Solution: Can list in a sequence

0, 1, − 1, 2, − 2, 3, − 3 ,………..

(A one-to-one correspondence with Z+)

Or can define a bijection from Z+ to Z:• When n is even: f(n) = n/2

• When n is odd: f(n) = −(n−1)/2

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Example 3: Show that the positive rational numbers are countable.Solution: The positive rational numbers are countable since they can be arranged in a sequence

Showing that a Set is Countable

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The Real Numbers are Uncountable

A proof by contradiction: called the Cantor diagonalization argument

1. Suppose R is countable. Then the real numbers between 0 and 1 are also countable (any subset of a countable set is countable - an exercise in the text).

2. The real numbers between 0 and 1 can be listed in order r1 , r2 , r3 ,… .3. Let the decimal representation of this listing be 1 11 12 13 14 15 16

2 21 22 23 24 25 26

0. ...0. ...

.

.

.

r d d d d d dr d d d d d d=

=4. Form a new real number with the decimalexpansion 1 2 3 4. ...r r r r r=where 3if 3 and 4if 3i ii i iir d r d= ¹ = =

5. r is not equal to any of the r1 , r2 , r3 ,... Because it differs from ri in its ith position after the decimal point. Therefore there is a real number between 0 and 1 that is not on the list since every real number has a unique decimal expansion. Hence, all the real numbers between 0 and 1 cannot be listed, so the set of real numbers between 0 and 1 is uncountable.

6. Since a set with an uncountable subset is uncountable (an exercise), the set of real numbers is uncountable.