Exercise 8.14

True or false? Explain.

  1. ${\exists\,x\;\;} x{Rumpelstiltskin}$ is a valid (necessarily true) sentence of first-order logic.

  2. Every existentially quantified sentence in first-order logic is true in any model that contains exactly one object.

  3. ${\forall\,x,y\;\;} xy$is satisfiable.

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