13. Given a non-empty set X, let be defined as . Show that the empty set is the identity for the operation * and all the elements A of P(X) are invertible with . ( Hint : and ).
13. Given a non-empty set X, let be defined as . Show that the empty set is the identity for the operation * and all the elements A of P(X) are invertible with . ( Hint : and ).
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