Monday, 5 June 2017

elementary set theory - Bijective function

Recently I was wondering if we need the Axiom of Choice in order to find an inverse function given an bijective funcion:

If f:AB is bijective we mean that f is injective and surjective. Assume that f:AB is bijective. I want to define f1:BA s.t. ff1=1B and f1f=1A. For each bB let Xb:={xX:f(x)=b}. By surjectivity each Xb is non-empty. But not knowing wether A or B are finite I need AC in order to select from each Xb an (in fact unique) element xb and defining f1(b):=xb for each bB.



Is AC necassary ?

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