Does Surjective-infinite implies Dedekind infinite

functionsset-theory

Suppose we define a set $A$ to be Surjective-infinite iff there is an surjection $f:A \rightarrow A$ such that it is not an injection. Is this true that $A$ is also dedekind-infinite, i.e there exist a $f^*$ from $A$ to $A$ that is injective but not surjective.

Conceptually, using AC if need be. I could use the given $f$ to construct $f^*$ by sort of 'removing' the duplicates from the pre-image of $a$ under $f$, meaning if $f(c)=f(b)=a,$ define $f^*(a)$ to just be $b$.

If this is true, how do I do it more concretely ?

Cheers

Best Answer

Assuming the axiom of choice, yes, quite easily. If $f\colon A\to A$ is a surjection, there is some $g\colon A\to A$ which is an injection and $f(g(a))=a$. Simply prove that since $f$ is not injective, $g$ cannot be surjective.

Not assuming choice, however, this is not necessarily true. It is consistent that there is a Dedekind-finite set $A$ and $f\colon A\to A$ which is surjective and not injective. For example, if $A$ is an infinite Dedekind-finite set, the set $S(A)$ of all finite injective sequences from $A$ is also Dedekind-finite and the function $f\colon S(A)\to S(A)$ given by erasing the last coordinate of a sequence is surjective but far from injective.

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