Function Definition Without a Single Word

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Discussion Overview

The discussion revolves around the definition of a function in set theory, specifically focusing on the notation and structure of defining a function as a set of ordered pairs. Participants explore the implications of circular definitions and the necessity of proper notation in set-builder definitions.

Discussion Character

  • Debate/contested
  • Technical explanation
  • Mathematical reasoning

Main Points Raised

  • One participant proposes a definition of a function as a set of ordered pairs but is challenged on the circularity of using the set itself in its definition.
  • Another participant argues that the definition is not circular, asserting that the use of the set in its definition does not imply recursion.
  • Concerns are raised about the lack of clarity in set-builder notation, specifically regarding what set the elements belong to.
  • Participants discuss the necessity of defining the set in terms of another established set, using the example of even integers to illustrate valid definitions.
  • There is a debate about whether the proposed definitions account for the existence of multiple functions from set A to set B.
  • Some participants express frustration over the lack of a clear, correct definition being provided amidst the discussion.
  • There are assertions that learning involves struggling with definitions and making mistakes, rather than simply receiving correct answers.

Areas of Agreement / Disagreement

Participants do not reach consensus on the correct definition of a function, with multiple competing views and ongoing disagreements about the validity of proposed definitions and the implications of circularity in definitions.

Contextual Notes

Participants highlight limitations in the definitions being discussed, particularly regarding the assumptions made about the existence and nature of the sets involved. The discussion reflects a complex interplay of notation and foundational concepts in set theory.

  • #31
dijkarte said:
Interesting approach. So there's one and only one universe that contains all possible mappings from sets A to B, and any f:A --> B is an element of this. I will research the topic further.

Well it's a set. You can get a handle on this by working out a simple example with small finite sets. For example let

A = {a1, a2, a3} and B = {b1, b2}. How many functions can there be from A to B?

Well a1 can go to b1 or b2.

a2 can go to b1 or b2.

a3 can go to b1 or b2.

So that gives us 2x2x2 = 8 possible functions from A to B. Each function is a set of exactly three ordered pairs, and there are only eight functions. Not too hard to write them all down.

Now you can see why we chose the notation B^A for the set of all functions from A to B. Because the set B^A has cardinality exactly |B|^|A|.
 

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