Inverting Injections and Surjections: A Discussion of the Axiom of Choice

  • MHB
  • Thread starter Swlabr1
  • Start date
  • Tags
    Injection
In summary, if there exists an injection from a set $X$ to a set $Y$, then there also exists a surjection from $Y$ to $X$ by choosing an element of $X$ and mapping the rest of the elements in $Y$ to it. However, the necessity of choosing this element raises concerns about the use of the axiom of choice. In ZFC, it is possible to use the axiom schema of specification and replacement to avoid invoking the axiom of choice in this situation. In finite cases, the function can be described without the need for the axiom of choice. However, with an uncountable set, the process of choosing an element becomes more complex and may require the use of the axiom of choice
  • #1
Swlabr1
15
0
If there exists an injection $f: X\rightarrow Y$ then there `obviously' exists a surjection $g:Y\rightarrow X$, got by noting that the restriction of $f$ to $f(X)$ is a bijection between $X$ and $f(X)$ and so invertible, and then mapping everything else in $Y$ to some fixed element in $X$. However, we have to `choose' this element, and I was wondering if there are any problems in doing this (w.r.t. the axiom of choice)?

If we do have to invoke AOC, is there any way of `inverting' an injection to get a surjection without using AOC?
 
Physics news on Phys.org
  • #2
I believe it is not needed here. Note that, given an injection from X to Y, we need to require that X is nonempty to be able to find a surjection from Y to X. Now, if follows from the axiom (or definition) of the empty set that a nonempty set has an element. This element can be used to map the rest of the elements of Y.

AC is important when when there is an infinite collection of nonempty sets and one has to build a single function that chooses an element of each set. If there is just one or finitely many nonempty sets, then such function can be described in a finite way.
 
  • #3
In ZFC, try to use Axiom schema of specification with Axiom schema of replacement (with Axiom schema of collection). I did not check, but it should work. Functions on sets are morphisms in set theory. My educated guess is that ZF (without the Axiom of choice) are strong enough to allow such an algebraically fundamental concept.
 
Last edited by a moderator:
  • #4
Evgeny.Makarov said:
I believe it is not needed here. Note that, given an injection from X to Y, we need to require that X is nonempty to be able to find a surjection from Y to X. Now, if follows from the axiom (or definition) of the empty set that a nonempty set has an element. This element can be used to map the rest of the elements of Y.

AC is important when when there is an infinite collection of nonempty sets and one has to build a single function that chooses an element of each set. If there is just one or finitely many nonempty sets, then such function can be described in a finite way.

i see a problem, here. what if X IS such a collection of non-empty sets? i sincerely doubt that there exists a way to select "any" element from an abitrary single set in a finite way. what if X is uncountable? the problem is: just calling X "a single set" does not mean X has a simple internal structure that can be finitely described.
 
  • #5
Deveno said:
i see a problem, here. what if X IS such a collection of non-empty sets? i sincerely doubt that there exists a way to select "any" element from an abitrary single set in a finite way. what if X is uncountable? the problem is: just calling X "a single set" does not mean X has a simple internal structure that can be finitely described.
Calling X "a single set" means that X is a set. On the other hand, "a collection of non-empty sets" need not be a set.

One of the basic ingredients in set theory is that the definition of a set has to be constrained (so as to avoid Russell's paradox), so that an arbitrary collection of elements is not necessarily a set. However, if X is a set, then it is legitimate, within ZF without C, to choose an element of X.

Given an injection $f:X\to Y$, it is implicit that the domain $X$ is a set. The OP's argument for constructing a surjection from $Y$ to $X$ (a left inverse for $f$, in fact) is valid in ZF with no need for AoC.
 
  • #6
i understand that a family of sets need not be a set itself. but suppose that our "X" is a Hamel basis for the real numbers over the rational field. X is a subset of the real numbers, and so it's clearly a set. there is a natural injection (inclusion) of this basis into the real numbers. in fact, we can consider the subset X' = X - {1}, and inclusion is still an injection. so we pick an element of X'...how?

with a countable set X, we can leverage the bijection and well-ordering of N to pick a "least" element of X. i am uncertain how one goes about generating a well-defined choice from an uncountable set. this may be sheer ignorance on my part, if so...please, enlighten me.
 

FAQ: Inverting Injections and Surjections: A Discussion of the Axiom of Choice

What is meant by "inverting" an injection?

"Inverting" an injection refers to the process of reversing the direction of an injected substance, typically in a laboratory setting. This can be done using specialized equipment or techniques, and is often used to study the effects of a substance on a specific system or to create a control for an experiment.

Why is "inverting" an injection important in scientific research?

Inverting an injection allows scientists to control and manipulate the direction of a substance in a controlled environment. This can help them better understand the effects of the substance and its interactions with other elements in the system. It also allows for the creation of a control in experiments, ensuring accurate and reliable results.

What substances are commonly "inverted" in scientific research?

Many different substances can be "inverted" in scientific research, including chemicals, drugs, and biological compounds. The specific substance used will depend on the research question and the system being studied.

What are some methods for "inverting" an injection?

There are several methods for inverting an injection, including using special needles or syringes, centrifugation, or specialized equipment such as microfluidic systems. The method used will depend on the substance being injected and the desired outcome.

Are there any risks associated with "inverting" an injection?

As with any scientific procedure, there are some potential risks associated with "inverting" an injection. For example, using improper equipment or techniques could lead to contamination or inaccurate results. It is important for scientists to follow proper protocols and safety measures when performing this procedure.

Similar threads

Back
Top