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Zermelo set theory

mathematics A set theory with the following set of

axioms:

Extensionality: two sets are equal if and only if they have

the same elements.

Union: If U is a set, so is the union of all its elements.

Pair-set: If a and b are sets, so is

a, b.

Foundation: Every set contains a set disjoint from itself.

Comprehension (or Restriction): If P is a formula with one

free variable and X a set then

is a set.

Infinity: There exists an infinite set.

Power-set: If X is a set, so is its power set.

Zermelo set theory avoids Russell's paradox by excluding

sets of elements with arbitrary properties - the Comprehension

axiom only allows a property to be used to select elements of

an existing set.