Why do the subsets in a partition have to be nonempty?

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Discussion Overview

The discussion revolves around the necessity for subsets in a partition of a set to be nonempty. Participants explore whether this requirement is a matter of convention or if it has deeper implications in mathematical definitions and applications.

Discussion Character

  • Conceptual clarification
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • Some participants assert that a partition must consist of nonempty subsets to adhere to the definition of a partition, suggesting that including empty sets would complicate many results related to partitions.
  • Others propose that while including the empty set in partitions is not mathematically incorrect, it diverges from standard definitions and practices.
  • One participant notes that considering partitions of the empty set leads to a unique partition, which is only valid if no empty subsets are included.
  • Another participant discusses the relevance of nonempty subsets in the context of defining Riemann or Lebesgue integrals, emphasizing that subsets must be of size greater than or equal to a positive epsilon to facilitate integration.
  • A later reply distinguishes between different types of partitions, indicating that while singleton subsets could be considered, they may not be suitable for integration purposes.

Areas of Agreement / Disagreement

Participants express differing views on the necessity of nonempty subsets in partitions, with some arguing for its importance based on definitions and applications, while others suggest that including empty sets could be permissible under certain conditions. The discussion remains unresolved regarding the implications of including empty subsets.

Contextual Notes

Limitations in the discussion include assumptions about the definitions of partitions and the implications of including empty sets, which are not fully explored. The scope is primarily focused on theoretical aspects rather than practical applications.

Ragnarok7
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"A partition of a set X is a set of nonempty subsets of X such that every element x in X is in exactly one of these subsets."

I was just wondering why the subsets must be nonempty. Is it just convention/convenient or is it because it would violate something else?

Thanks!
 
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The empty set does not belong to any set of equivalence classes.
 
Ragnarok said:
"A partition of a set X is a set of nonempty subsets of X such that every element x in X is in exactly one of these subsets."

I was just wondering why the subsets must be nonempty. Is it just convention/convenient or is it because it would violate something else?

Thanks!
I think it's just a matter of definition.

Certainly, if you don't follow this, many results about partitions will need to be tweaked.
There is nothing mathematically wrong in including the empty set also when considering partitions but that is not how it has been done.
 
Partitions are used to generate equivalence relations on a set, and vice versa.

It is sometimes useful to consider partitions of the empty set itself; intuitively one feels that this partition should be unique, which is only possible if there are NO possible empty sets in a partition (convince yourself that this indeed comprises the only partition possible in the definition you gave for an empty set X).
 
Typically partitions are used to define a Riemann or Lebesgue integral.
For that to work, we need to be able to define a limit where the partition becomes infinitely fine grained, meaning each subset in the partition should be of size greater than or equal to $\varepsilon >0$.
In other words: not empty.
 
I like Serena said:
Typically partitions are used to define a Riemann or Lebesgue integral.
For that to work, we need to be able to define a limit where the partition becomes infinitely fine grained, meaning each subset in the partition should be of size greater than or equal to $\varepsilon >0$.
In other words: not empty.

Wrong kind of partition: the kind the OP is talking about might be a partition into singleton subsets, which would be fine...but such a partition would be rather unsuitable for integration.
 

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