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Week 1 - Wednesday

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Course overview Propositional logic Truth tables AND, OR, NOT Logical equivalence

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You come to a fork in the road Two men stand beneath a sign that reads: Ask for the way, but waste not your breath One road is freedom, the other is death Just one of the pair will lead you aright For one is a Knave, the other a Knight What single yes or no question can you ask to determine which fork to take?

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What’s an expression that logically equivalent to ~(p q) ? What about logically equivalent to ~(p q) ? De Morgan’s Laws state: ~(p q) ~p ~q ~(p q) ~p ~q Essentially, the negation flips an AND to an OR and vice versa

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A tautology is something that is true no matter what Examples: TT p ~p p pp p The final column in a truth table for a tautology is all true values The book sometimes writes a statement which is a tautology as a t

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A contradiction is something that is false no matter what Examples: FF p ~p ~(p p) The final column in a truth table for a contradiction is all false values The book sometimes writes a statement which is a contradiction as a c

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NameLawDual Commutative p q q pp q q pp q q pp q q p Associative (p q) r p (q r)(p q) r p (q r) Distributive p (q r) (p q) (p r)p (q r) (p q) (p r) Identity p t pp c p Negation p ~p tp ~p c Double Negative ~(~p) p Idempotent p p pp p pp p pp p p Universal Bound p t tp t tp c c De Morgan’s ~(p q) ~p ~q~(p q) ~p ~q Absorption p (p q) pp (p q) p Negations of t and c ~t c~t c~c t~c t

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You can construct all possible outputs using combinations of AND, OR, and NOT But, sometimes it’s useful to introduce notation for common operations This truth table is for p q pq p qp q TTT TFF FTT FFT

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We use to represent an if-then statement Let p be “The moon is made of green cheese” Let q be “The earth is made of rye bread” Thus, p q is how a logician would write: If the moon is made of green cheese, then the earth is made of rye bread Here, p is called the hypothesis and q is called the conclusion What other combination of p and q is logically equivalent to p q ?

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p q is true when: p is true and q is true p is false Why? For the whole implication to be true, the conclusion must always be true when the hypothesis is true If the hypothesis is false, it doesn’t matter what the conclusion is “If I punch the tooth fairy in the face, I will be Emperor of the World” What’s the negation of an implication?

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Given a conditional statement p q, its contrapositive is ~q ~p Conditional: “If a murderer cuts off my head, then I will be dead.” Contrapositive: “If I am not dead, then a murderer did not cut off my head.” What’s the relationship between a conditional and its contrapositive?

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Given a conditional statement p q: Its converse is q p Its inverse is ~p ~q Consider the statement: “If angry ham sandwiches explode, George Clooney will become immortal.” What is its converse? What is its inverse? How are they related?

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Sometimes people say “if and only if”, as in: “A number is prime if and only if it is divisible only by itself and 1.” This can be written p iff q or p q This is called the biconditional and has this truth table: What is the biconditional logically equivalent to? pq p qp q TTT TFF FTF FFT

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An argument is a list of statements (called premises) followed by a single statement (called a conclusion) Whenever all of the premises are true, the conclusion must also be true, in order to make the argument valid

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Are the following arguments valid? p q ~r(premise) q p r(premise) p q(conclusion) p (q r)(premise) ~r(premise) p q(conclusion)

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Modus ponens is a valid argument of the following form: p qp q pp q q Modus tollens is a contrapositive reworking of the argument, which is also valid: p qp q ~q~q ~p ~p Give verbal examples of each We call these short valid arguments rules of inference

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The following are also valid rules of inference: pp p q p q qq p q p q English example: “If pigs can fly, then pigs can fly or swans can breakdance.”

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The following are also valid rules of inference: p q p p q q English example: “If the beat is out of control and the bassline just won’t stop, then the beat is out of control.”

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The following is also a valid rule of inference: pp qq p q English example: “If the beat is out of control and the bassline just won’t stop, then the beat is out of control and the bassline just won’t stop.”

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The following are also valid rules of inference: p q ~q~q p p ~p~p q q English example: “If you’re playing it cool or I’m maxing and relaxing, and you’re not playing it cool, then I’m maxing and relaxing.”

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The following is also a valid rule of inference: p q q rq r p r English example: “If you call my mom ugly I will call my brother, and if I call my brother he will beat you up, then if you call my mom ugly my brother will beat you up.”

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The following is also a valid rule of inference: p q p r q rq r r r English example: “If am fat or sassy, and being fat implies that I will give you trouble, and being sassy implies that I will give you trouble, then I will give you trouble.”

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The following is also a valid rule of inference: ~p c p p English example: “If my water is at absolute zero then the universe does not exist, thus my water must not be at absolute zero.”

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A fallacy is an argument that is not valid It could mean that the conclusion is not true in only a single case in the truth table But, if the conclusion is ever false whenever all the premises are true, the argument is a fallacy Most arguments presented by politicians are fallacies for one reason or another

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Converse error If Joe sings a sad song, then Joe will make it better. Joes makes it better. Conclusion: Joe sings a sad song. FALLACY Inverse error If you eat too much, you will get sick. You are not eating too much. Conclusion: You will not get sick. FALLACY

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Digital logic circuits are the foundation of all computer hardware Circuits are built out of components called gates A gate has one or more inputs and an output Gates model Boolean operations Usually, in digital logic, we use a 1 for true and a 0 for false

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The following gates have the same function as the logical operators with the same names: NOT gate: AND gate: OR gate:

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Draw the digital logic circuit corresponding to: (p ~q) ~(p r) What’s the corresponding truth table?

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Predicate logic Universal quantifier Existential quantifier

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Read Chapter 3

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