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Axiom ax-rep 4520
Description: Axiom of Replacement. An axiom scheme of Zermelo-Fraenkel set theory. Axiom 5 of [TakeutiZaring] p. 19. It tells us that the image of any set under a function is also a set (see the variant funimaex 5615). Although may be any wff whatsoever, this axiom is useful (i.e. its antecedent is satisfied) when we are given some function and encodes the predicate "the value of the function at is ." Thus, will ordinarily have free variables and - think of it informally as ( , ). We prefix with the quantifier A.y in order to "protect" the axiom from any containing , thus allowing us to eliminate any restrictions on . Another common variant is derived as axrep5 4525, where you can find some further remarks. A slightly more compact version is shown as axrep2 4522. A quite different variant is zfrep6 6678, which if used in place of ax-rep 4520 would also require that the Separation Scheme axsep 4529 be stated as a separate axiom.

There is very a strong generalization of Replacement that doesn't demand function-like behavior of . Two versions of this generalization are called the Collection Principle cp 8235 and the Boundedness Axiom bnd 8236.

Many developments of set theory distinguish the uses of Replacement from uses the weaker axioms of Separation axsep 4529, Null Set axnul 4537, and Pairing axpr 4647, all of which we derive from Replacement. In order to make it easier to identify the uses of those redundant axioms, we restate them as axioms ax-sep 4530, ax-nul 4538, and ax-pr 4648 below the theorems that prove them. (Contributed by NM, 23-Dec-1993.)

Assertion
Ref Expression
ax-rep
Distinct variable group:   , , ,

Detailed syntax breakdown of Axiom ax-rep
StepHypRef Expression
1 wph . . . . . . 7
2 vy . . . . . . 7
31, 2wal 1368 . . . . . 6
4 vz . . . . . . 7
54, 2weq 1696 . . . . . 6
63, 5wi 4 . . . . 5
76, 4wal 1368 . . . 4
87, 2wex 1587 . . 3
9 vw . . 3
108, 9wal 1368 . 2
114, 2wel 1759 . . . . 5
12 vx . . . . . . . 8
139, 12wel 1759 . . . . . . 7
1413, 3wa 369 . . . . . 6
1514, 9wex 1587 . . . . 5
1611, 15wb 184 . . . 4
1716, 4wal 1368 . . 3
1817, 2wex 1587 . 2
1910, 18wi 4 1
Colors of variables: wff setvar class
This axiom is referenced by:  axrep1  4521  axnulALT  4536  bj-axrep1  33154  bj-snsetex  33301
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