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The [ilmath]\exists[/ilmath]-functor is a covariant functor taking [ilmath]\mathrm{SET} [/ilmath] [ilmath]\leadsto[/ilmath] [ilmath]\mathrm{SET} [/ilmath] defined as follows[1]:

  • [ilmath]\forall A\in\text{Ob}(\mathrm{SET})[\exists:A\mapsto\mathcal{P}(A)][/ilmath], recall [ilmath]\mathcal{P}(X)[/ilmath] denotes the power set of [ilmath]X[/ilmath]
    • The functor sends all objects to their powerset. and
  • [ilmath]\forall (f:A\rightarrow B)\in\text{Mor}(\mathrm{SET})\left[\exists(f)\mapsto\left\{\begin{array}{l}:\mathcal{P}(A)\rightarrow\mathcal{P}(B)\\:X\mapsto f(X)\end{array}\right.\right][/ilmath]
    • That is to say the [ilmath]\exists[/ilmath]-functor takes the function [ilmath]f[/ilmath] to a function that takes each element of the powerset of the domain of [ilmath]f[/ilmath] to the image of that element under [ilmath]f[/ilmath] (which is obviously in the powerset of the co-domain of [ilmath]f[/ilmath])

Proof of claim

Claim: that the [ilmath]\exists[/ilmath]-functor is a functor

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See page 82 in[1]


Notice that the [ilmath]\exists[/ilmath]-functor has the following property:

  • Given a [ilmath](f:A\rightarrow B)\in\text{Mor}(\mathrm{SET})[/ilmath]
  • [ilmath]\forall X\in\mathcal{P}(A)\ \forall b\in B\Big[ [b\in(\exists(f))(X)]\iff[\exists a\in A(b=f(a)\wedge a\in X)]\Big][/ilmath]

TODO: Describe this, something like [ilmath]b[/ilmath] is in the image of [ilmath]X[/ilmath] under the image of [ilmath]f[/ilmath] in the functor if and only if there is an [ilmath]a\in X[/ilmath] such that [ilmath]f(a)=b[/ilmath]

See also

TODO: A forall-exists-inverse functor page comparison


  1. 1.0 1.1 An Introduction to Category Theory - Harold Simmons - 1st September 2010 edition