“In set theory, the complement of a set A refers to elements not in A.”
If A is a subset* of a given set U, then the absolute complement of A is the set of elements that are in U but not in A. Note that U may be defined only implicitly.
If A and B are both subsets* of a given set U, then “the relative complement of A with respect to a set B, also [called] the set difference of B and A, written B \ A, is the set of elements [that are] in B but not in A.“
Quotes and information taken from “Complement (set theory),” Wikipedia, retrieved 6/17/2020, emphasis added. Note that, as of retrieving this information, there are some pictures there that could be useful in understanding this.
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*What is the Gist of a “Subset”?
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Disclaimer:
I am not a professional in this field, nor do I claim to know all of the jargon that is typically used in this field. I am not summarizing my sources; I simply read from a variety of websites until I feel like I understand enough about a topic to move on to what I actually wanted to learn. By definition, none of these posts address every aspect of a topic. If I am inaccurate in what I say or you know a better, simpler way to explain a concept, I would be happy to hear from you :).
