1. Compute the values: D2-2D1, D3-3D2, D4-4D3, D5-5D4, …, D10-10D9 2.What is the pattern?
1. Compute the values: D2-2D1, D3-3D2, D4-4D3, D5-5D4, …, D10-10D9 2.What is the pattern?
Database System Concepts
7th Edition
ISBN:9780078022159
Author:Abraham Silberschatz Professor, Henry F. Korth, S. Sudarshan
Publisher:Abraham Silberschatz Professor, Henry F. Korth, S. Sudarshan
Chapter1: Introduction
Section: Chapter Questions
Problem 1PE
Related questions
Question
1. Compute the values: D2-2D1, D3-3D2, D4-4D3, D5-5D4, …, D10-10D9
2.What is the pattern?

Transcribed Image Text:8.1 Examples Defined by Recurrence Equations
Example 8.1.1: Derangements
Imagine a party where couples arrive together, but at the end of the evening,
every person leaves with a new partner. For each n E P, let D„ denote the number of
different ways n couples can be "deranged" – that is, rearranged in couples so no
one is paired with the person they arrived with.
// one couple cannot be deranged.
/3 one and only one way to derange two couples.
// If the couples arrive as Aa, Bb, and Cc,
// then A would be paired with b or c.
//
Then D1 = 0
D2
D3 = 2
If A is paired with b, C must be paired with a (not c),
and then B with c.
//
//
If A is paired with
C,
B must be paired with a (not b),
//
and then C with b.
// How big are D4, D5, and D10? How can we calculate them? Is there a formula?
Let's develop a strategy for counting derangements when n >= 4. Suppose the
n women are A1, A2, A3,
Woman A1 may be "re-paired" with any of the n –
let's say she's paired with az where 2 <= k <= n. Now let's consider az's original
partner, woman Ag: she might take a, or she might refuse a1 and take someone else.
If A1 is paired with ar and Ak is paired with a1, there are n
derange, and that may be done in exactly D,–2
If Aj is paired with ak but Ak refuses a1, we could pretend that Ak and a1
together, so now, there are n
exactly Dn-1 different ways.
For each of the n –
An, and each A; arrives with man a;.
1 men az or az or . . . or an;
2 couples left to
different
ways.
arrived
1 couples left to derange which may be done in
1 men that A1 might choose, there are {Dn-2 + Dn-1}
different ways to complete the derangement. Therefore, when n >= 4,
D, = (n – 1){Dn-2+Dn-1}.
(8.1.2)
// This also applies when n =
// Equation 8.1.2 is an example of a second-order recurrence equation,
// where the generic entry is expressed in terms the previous two entries.
3.
![Using Eq. 8.1.2 together with the values of D, and D2, we can evaluate D, for
any value of n:
// in principle
D3
D4
D5
D6
D7
D8
D9
D10
(3 – 1){D2+D1} = 2{1+0}
(4 – 1){D3 +D2} = 3{2+1}
(5 – 1){D4+D3} = 4{9+2}
(6 – 1){D5+D4} = 5{44+9}
(7 – 1){D6+D5} = 6{265 +44}
(8 – 1){D7+D6} = 7{1854+265}
(9 – 1){D8+D7} = 8{14833+1854}
(10 – 1){D9+Ds} = 9{133496 +14833}
2
44
265
1 854
14 833
133 496
1 334 961
// It's strange that 1 334 961 = 10 × (133 496) + 1. Or is it?
// Is there a (convenient and compact) formula for D, that we can use to calculate
// its values?
The sequence on P defined by S,= A × n! where A is any real number satisfies
the recurrence equation (8.1.2). If n >= 3 then
(n – 1){S„-2+Sn-1} = (n – 1){A(n – 2)! +A(n – 1)!}
(n – 1)A(n – 2)!{1+[n – 1]}
А(п — 1) (п — 2)!{n}
= A x n!
= Sn.
%3|
||
// But will this "formula" apply when n =
// Does there exist a real number A such that D,
// No, because
// and
1 or n =
2?
A(n!) when n = 1 or n = 2?
if 0 = D1 = A(1!), then A must equal 0,
if 1 =
D2 = A(2!), then A must equal ½.
We can however use this formula to prove that D, is O(n!) by proving
Theorem 8.1.1: For all n >= 2, (1/3)n! <= Dn <= (1/2)n!
Consider the table of values
n
(1/3)n!
Dn
(1/2)n!
1
1/3
1/2
2
2/3
1
1 = 2/2
3
6/3 = 2
2
3 = 6/2
4
24/3 = 8
9.
12 = 24/2
120/3 = 40
44
60
= 120/2
6.
720/3 = 240
265
360 = 720/2
|| || || ||| || ||||](/v2/_next/image?url=https%3A%2F%2Fcontent.bartleby.com%2Fqna-images%2Fquestion%2Fa4792360-22b4-40c0-a48a-02ce03c9f6f7%2F0961cc65-14c2-4cc7-92ce-be168c51704c%2Fxms5tjf_processed.png&w=3840&q=75)
Transcribed Image Text:Using Eq. 8.1.2 together with the values of D, and D2, we can evaluate D, for
any value of n:
// in principle
D3
D4
D5
D6
D7
D8
D9
D10
(3 – 1){D2+D1} = 2{1+0}
(4 – 1){D3 +D2} = 3{2+1}
(5 – 1){D4+D3} = 4{9+2}
(6 – 1){D5+D4} = 5{44+9}
(7 – 1){D6+D5} = 6{265 +44}
(8 – 1){D7+D6} = 7{1854+265}
(9 – 1){D8+D7} = 8{14833+1854}
(10 – 1){D9+Ds} = 9{133496 +14833}
2
44
265
1 854
14 833
133 496
1 334 961
// It's strange that 1 334 961 = 10 × (133 496) + 1. Or is it?
// Is there a (convenient and compact) formula for D, that we can use to calculate
// its values?
The sequence on P defined by S,= A × n! where A is any real number satisfies
the recurrence equation (8.1.2). If n >= 3 then
(n – 1){S„-2+Sn-1} = (n – 1){A(n – 2)! +A(n – 1)!}
(n – 1)A(n – 2)!{1+[n – 1]}
А(п — 1) (п — 2)!{n}
= A x n!
= Sn.
%3|
||
// But will this "formula" apply when n =
// Does there exist a real number A such that D,
// No, because
// and
1 or n =
2?
A(n!) when n = 1 or n = 2?
if 0 = D1 = A(1!), then A must equal 0,
if 1 =
D2 = A(2!), then A must equal ½.
We can however use this formula to prove that D, is O(n!) by proving
Theorem 8.1.1: For all n >= 2, (1/3)n! <= Dn <= (1/2)n!
Consider the table of values
n
(1/3)n!
Dn
(1/2)n!
1
1/3
1/2
2
2/3
1
1 = 2/2
3
6/3 = 2
2
3 = 6/2
4
24/3 = 8
9.
12 = 24/2
120/3 = 40
44
60
= 120/2
6.
720/3 = 240
265
360 = 720/2
|| || || ||| || ||||
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