The theory governing these objects was developed by Zermelo and Fraenkel in the early 20th century to avoid a paradox discovered by Bertrand Russell. The size of one of these objects is called its cardinality, and the (*) intersection of two of these objects yields anything that is found in both of 
The theory governing these objects was developed by Zermelo and Fraenkel in the early 20th century to avoid a paradox discovered by Bertrand Russell. The size of one of these objects is called its cardinality, and the (*) intersection of two of these objects yields anything that is found in both of them. These objects are unordered and do not include any repeated elements. For 10 points, give this mathematical term for a collection of objects.
The theory governing these objects was developed by Zermelo and Fraenkel in the early 20th century to avoid a paradox discovered by Bertrand Russell.
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