Transcript Lecture 5 Knights and Knaves.

Lecture 5 Knights and Knaves.
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The island of Knights and Knaves.
• The island of Knights and Knaves is a fictional
island to test peoples’ ability to reason logically.
• There are two types of people on the island.
• Knights who always tell the truth
• Knaves who always lie.
• Logic puzzles are about deducing facts about the
island, from statements made by the people on
the island. But of course, you do not know
whether the statement was made by a knight or a
knave. I am a knight, I am a knave!!!!!
Some examples
• What happens if you ask a person if they are a knight
or a knave?
• There may be gold on the island. What question could
you ask to establish this?
• A native says “there is gold on this island, is the same
as, I am a knight”.
• What question would you ask a pair of natives to
discover if the other one is a knight?
• A tourist comes to a fork in a road, one way leads to a
restaurant, the other does not. What question (yes/no)
would you ask a native to find out which way it is to
the restaurant?
Algebra
• At school we learnt to manipulate expressions.
The values of expressions are numbers
• n^2 –m^2 = (n + m)(n - m)
• To do this we use laws.
• e.g. n + 0 = n and n –n = 0 (for ANY values of n)
• Associatively of addition
• (m + n) +p = m + (n + p)
• I am sure you are all familiar with laws regarding
addition, multiplication.
Boolean algebra
• Boolean expressions are either true or false.
• Boolean valued expressions are called
propositions.
• “it is sunny” is an atomic Boolean expression.
• “it is sunny and warm” is non atomic as it can
be broken down into two expressions.
• We are concerned with the rules for
manipulating Boolean expressions.
Knights and knaves.
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“A” is a native on the island.
And is therefore either a knight or a knave.
The statement “A is a knight” is either true or false.
The statement “the restaurant is to the left” is either
true or false.
• Suppose “A is a knight”, and suppose A makes
statement S.
• The crucial observation is that the values of these two
propositions are the same.
• Then “A is a knight” = S. Draw the truth table. P65
Example
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If A says “the restaurant is to the left” then
“A is a knight”=”the restaurant is to the left”
In words,
A is a knight and the restaurant is to the left,
or,
A is not a knight and the restaurant is not to
the left
Yes No response
• If a native A is asked a question Q, the
response will be the truth value of A=Q.
• The response will be “yes” if A is a knight and
the answer is really yes. Or
• A is a knave and the answer is really no.
• Otherwise the response will be “no”.
Simple example p66
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A is asked whether B is a knight. And
B is asked whether A is a knight.
A’s response is A=B
B’s response is B=A
Equality is symmetrical so the two responses
will always be the same.
• Note that this does not involve case analysis
of the four cases (draw truth table)
5.2.3 Boolean Equality.
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5.2.3 Boolean Equality.
Equality is reflexive. x=x
It is symmetric x=y is the same as y=x
It is transitive x=y and y=z implies z=x
Finally x=y, then f(x) = f(y)
Equality is a binary relation.
It is a function with a range of Boolean values
true and false.
Is equality associative?
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Multiplication and addition are associative.
Multiplication and addition are symmetric.
Is equality associative?
The expression (p = q) = r does not make
sense if p q r are numbers, characters,
sequences or objects. Only for Boolean values.
• It makes sense to ask if equality of Boolean
values is associative.
Associatively
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((p = q) = r) = (p = (q = r))
Construct a truth table.
It is true if an odd number of atoms are true.
(p=p)=true and p= (p=true)
The 2nd is used for elimination.
Hidden treasures. Yes/No question
• A native says “I am a knight equals there is
gold on the island”
• Let A represent “the native is a knight”
• Let G represent “there is gold on the island”
• The native’s statement is A=G so we deduce
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A = (A = G)
{equality of Booleans is associative}
(A=A)=G
{(A=A) = true}
True = G
{Equality is symmetrical}
G=true
{G = (G=true)}
G
We conclude that there is gold on the island, but we
cannot deduce if the native is a knight or a knave.
What question to ask.
• You are at a fork in the road, you want to know if the gold is to the
left or the right.
• Let Q be the question to be posed.
• The response to the question will be A = Q
• Let L denote, “the gold can be found by following the left fork”
• The requreimet is that L is the same are the response to Q.
• i.e. we reqire L=(A=Q)
• {but as equality is associative}
• (L = A) = Q
• So the question Q posed is L=A
• Ie “is the value of ‘the gold can be found by following the left fork’
equal to the value of ‘you are a knight’”
Equals for Equals
• There are three natives A B C.
• C says “A and B are both the same type”.
• Formulate a question, that when posed to A determines if C is
telling the truth.
• Let A be the statement A is a “knight”.
• Let Q be the unknown question.
• The response we want is C (i.e. if C is true then C is a knight).
• By the previous section, Q = (A=C) i.e. we replace L by C.
• C’s statement is A=B, so now we know C = (A=B) by equality.
• So Q = (A = (A=B)) which simplifies to Q = B, so the question to be
posed is “is B a knight”. Show the formal working from book p 69
Equivalence and continued Equalities
• Do we understand x=y=z as
• (x=y)=z or as x=(y=z)
• The two reading are different, why? Do
calculation on board p71
• Explain subtle difference on board.
• We pronounce this as equivales
Examples of the Associatively of
Equivalence
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A beautiful example.
n + m is even equivales n is even equivales m is even
We refer to “is even” of a number as parity.
n + m is even equivales (n is even equivales m is even)
states n + m is even when the parities of m and n are the
same.
(n + m is even equivales n is even) equivales m is even
States, the operation of adding a number n to a number m
does not change the parity of m exactly when n is even.
Another way to read the statement is see page 72.
This single statement avoids case analysis (four in this case).
Negation 1
• Two natives A B
• Native A says “B is a knight equals I am not a
knight”
• If A makes a statement S we know A=S, so we
get A = (B=not A)
• (B=not A) means B is a knight equals I am not
a knight.
Negation 2
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not p = p= false
Read as not p = (p = false)
Functions as a definition, i.e. what does not p
mean, it means p is false.
Read as (not p = p) = false
Is useful for simplifying in expressions.
Go thru algebra on p75
Simplification
• The law of negation, along with associative
and symmetry gives us a way to simplify
expressions
• Do e.g. on page 75.
• Proof not false = true
Inequivalence. P80
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A says “B is different from myself”.
Formulate as B not= A
Or as not (B=A)
This is the same as “B is a knight equals I am
not a knight”
• Show working p80
• Inequivalence not(p=q)= p = not q
• Show working p80