What is the smallest closed subset of Z containing 2 and 0?

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Discussion Overview

The discussion centers on identifying the smallest closed subset of the integers (Z) that contains the elements 0 and 2, specifically focusing on the concept of closure under addition.

Discussion Character

  • Conceptual clarification, Debate/contested

Main Points Raised

  • One participant proposes that the smallest closed subset A could be {0, 2}, arguing that it includes both the identity element and the given element.
  • Another participant counters this by explaining the definition of closure under addition, stating that A = {0, 2} is not closed because adding the elements (2 + 2) results in 4, which is not in the set.
  • A third participant reiterates the definition of closure, emphasizing that a set must include the sum of any two of its elements to be considered closed.
  • A later reply acknowledges the oversight in the initial proposal, indicating a recognition of the error.

Areas of Agreement / Disagreement

Participants generally disagree on the initial claim regarding the smallest closed subset, with one viewpoint being corrected by others who clarify the requirements for closure under addition.

Contextual Notes

The discussion highlights the importance of understanding closure properties in set theory, particularly in relation to binary operations, but does not resolve the broader implications of closure in different contexts.

Who May Find This Useful

This discussion may be useful for individuals interested in set theory, mathematical operations, and the properties of closed sets in the context of integers.

Gear300
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The question given is: Determine the smallest subset A of Z such that 2 ε A and A is closed with respect to addition.

The answer given was the set of all positive even integers, but I was thinking that the smallest subset would be the given element and the identity element (0 in this case) so that A = {0,2}...wouldn't this be more accurate?
 
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To understand why that is not correct, you have to know what closure of an operator over a set means.

A set S is closed under a binary operator + iff for all x, y in S, x + y is in S.

A = {0, 2} isn't closed under addition because the definition is not satisfied. We can find a counter example where x and y are both in S, but x + y is not. The counter example is with x = 2 and y = 2.
 
Closed means you can do it to any two (possibly non-unique) elements and get an answer in your set.

S = {0, 2} doesn't work since 2+2=4 is not in S.
 
I see. Thanks...funny I didn't notice that.
 

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