Foundations of Mathematics

Set Theory

What are the basic symbols used to operate on sets?

When set theory founder Georg Cantor developed the symbols for sets, he used a single horizontal overbar to denote a set with no structure besides order; thus, it represented the order type of the set. A double bar meant that there was no order from the set, also called the cardinal number of the set (see below).

The more common symbols accepted in today’s set theory include the following:



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