Questions 7-8 Pre-Write

7. Write an official WFF expressing the Empty Set Axiom
Unofficial WFF:
∃y(∀z zЄy)
There exists a y such that for all z, z is not an element of y.
Possible official WFF:
∃y∀z¬(z Є y)

8. Intersection WFF
Statement: Given a set, there is a set ∩x consisting of all elements that belong to x.
∩x = ∀z(zЄy ↔ z belongs to every w in x)
= ∀z(zЄy ↔ ∀w(wЄx → zЄw)
Suppose that x=∅. For the formula above, inputting the empty set for x gives the result ∩∅=(zЄy), which is obviously incorrect. To account for this, we can rewrite the formula as:
∩x = ∀z(zЄy ↔ (x≠∅) ∧ (∀w(wЄx → zЄw)))
Similarly, ∩ N returns the empty set. This is incorrect as N itself is not empty, only its elements are.

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