Subobject Classifier - Definition

Definition

For the general definition, we start with a category C that has a terminal object, which we denote by 1. The object Ω of C is a subobject classifier for C if there exists a morphism

1 → Ω

with the following property:

for each monomorphism j: U → X there is a unique morphism χ j: X → Ω such that the following commutative diagram
is a pullback diagram — that is, U is the limit of the diagram:

The morphism χ j is then called the classifying morphism for the subobject represented by j.

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