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Axiom ax-un 4118
Description: Axiom of Union. An axiom of Intuitionistic Zermelo-Fraenkel set theory. It states that a set y exists that includes the union of a given set x i.e. the collection of all members of the members of x. The variant axun2 4120 states that the union itself exists. A version with the standard abbreviation for union is uniex2 4121. A version using class notation is uniex 4122.

This is Axiom 3 of [Crosilla] p. "Axioms of CZF and IZF", except (a) unnecessary quantifiers are removed, (b) Crosilla has a biconditional rather than an implication (but the two are equivalent by bm1.3ii 3851), and (c) the order of the conjuncts is swapped (which is equivalent by ancom 253).

The union of a class df-uni 3554 should not be confused with the union of two classes df-un 2898. Their relationship is shown in unipr 3567. (Contributed by NM, 23-Dec-1993.)

Assertion
Ref Expression
ax-un yz(w(z w w x) → z y)
Distinct variable group:   x,w,y,z

Detailed syntax breakdown of Axiom ax-un
StepHypRef Expression
1 vz . . . . . . 7 setvar z
2 vw . . . . . . 7 setvar w
31, 2wel 1376 . . . . . 6 wff z w
4 vx . . . . . . 7 setvar x
52, 4wel 1376 . . . . . 6 wff w x
63, 5wa 97 . . . . 5 wff (z w w x)
76, 2wex 1363 . . . 4 wff w(z w w x)
8 vy . . . . 5 setvar y
91, 8wel 1376 . . . 4 wff z y
107, 9wi 4 . . 3 wff (w(z w w x) → z y)
1110, 1wal 1226 . 2 wff z(w(z w w x) → z y)
1211, 8wex 1363 1 wff yz(w(z w w x) → z y)
Colors of variables: wff set class
This axiom is referenced by:  zfun  4119  axun2  4120  bj-axun2  6482
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