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Propositional Logic

Logical connectives, truth tables and logical equivalence, resuming the knights-and-knaves puzzles now solvable by truth table instead of trial and error.

Discrete Mathematics: An Open Introduction, Propositional Logic

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4 exercises

Exercise 1

Consider the statement about a party, “If it's your birthday or there will be cake, then there will be cake.”

  1. Translate the above statement into symbols. Clearly state which statement is PP and which is QQ.

  2. Make a truth table for the statement.

  3. Assuming the statement is true, what (if anything) can you conclude if there will be cake?

  4. Assuming the statement is true, what (if anything) can you conclude if…

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Exercise 6

Determine whether the following two statements are logically equivalent: ¬(P→Q)\neg(P \imp Q) and P∧¬QP \wedge \neg Q. Explain how you know you are correct.

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Exercise 8

Simplify the following statements (so that negation only appears right before variables).

  1. ¬(P→¬Q)\neg(P \imp \neg Q).
  2. (¬P∨¬Q)→¬(¬Q∧R)(\neg P \vee \neg Q) \imp \neg (\neg Q \wedge R).
  3. ¬((P→¬Q)∨¬(R∧¬R))\neg((P \imp \neg Q) \vee \neg (R \wedge \neg R)).
  4. It is false that if Sam is not a man then Chris is a woman, and that Chris is not a woman.

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Exercise 16

We can also simplify statements in predicate logic using our rules for passing negations over quantifiers, and then applying propositional logical equivalence to the “inside” propositional part. Simplify the statements below (so negation appears only directly next to predicates).

  1. ¬∃x∀y(¬O(x)∨E(y))\neg \exists x \forall y (\neg O(x) \vee E(y)).
  2. ¬∀x¬∀y¬(x<y∧∃z(x<z∨y<z))\neg \forall x \neg \forall y \neg(x \lt y \wedge \exists z (x \lt z \vee y \lt z)).
  3. There is a number nn for which no other number is either less nn than or equal to nn.

  4. It is false that for every number

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Adapted from Discrete Mathematics: An Open Introduction © 2013–2019 Oscar Levin. Licensed under CC BY-SA 4.0. Source: Discrete Mathematics: An Open Introduction. Imported at revision 1e2e26b.