Talk:Disjoint union

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Mathematics rating:	Stub Class	Mid Priority	Field: Foundations, logic, and set theory
Please update this rating as the article progresses, or if the rating is inaccurate. Click to show/hide comments. Please add to or update the comments to suggest improvements to the article. A section heading and a reference or two would easily lift this into start class. Geometry guy 20:47, 20 May 2007 (UTC)

[edit] I don't know what a disjoint union of sets is.

According to this Open University article, "In general, the disjoint union of sets X and Y ... is the set consisting of all items that are either from X or from Y [but not both]", so it's an exclusive-or operation on sets?

Kfor (talk) 20:02, 1 February 2008 (UTC)

It's not the same as an exclusive-or: if X and Y are sets with cardinality a and b repsectively, then their disoint union always has cardinality a+b. The OU article you refer to has two sets that have no elements in common (empty intersection - so they are disjoint sets), and the disjoint union of two sets is the ordinary union if they are already disjoint. The difficulty arises when the two sets are not disjoint - so to form their disjoint union some trick must be played. There are several ways to describe this, and the article does one of those. But I do think it could be clearer. Simplifix (talk) 23:10, 13 February 2008 (UTC)