What is Set Theory?

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  • Written By: Michael Anissimov
  • Edited By: R. Kayne
  • Last Modified Date: 25 September 2017
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Set theory constitutes most of the foundation of modern mathematics, and was formalized in the late 1800s. Set theory describes some very fundamental and intuitive ideas about how things called "elements" or "members" fit together into groups. Despite the apparent simplicity of the ideas, set theory is quite rigorous. In seeking to eliminate all arbitrariness in their theories, mathematicians have fine-tuned set theory to an impressive degree over the years.

In set theory a set is any well-defined group of elements or members. Sets are usually symbolized by italicized capital letters like A or B. If two sets contain the same members, they can be shown as equivalent with an equal sign.

The contents of a set can be described in simple English: A = all terrestrial mammals. Contents can also be listed within brackets: A = {bears, cows, pigs, etc.} For large sets, ellipsis may be employed, where the pattern of the set is obvious. For example, A = {2, 4, 6, 8... 1000}. One type of set has zero members, the set known as the empty set. It is symbolized by a zero with a diagonal line ascending left to right. Though seemingly trivial, it turns out to be quite important mathematically.


Some sets contain other sets, therefore being labeled supersets. The contained sets are subsets. In set theory, this relationship is referred to as "inclusion" or "containment," symbolized by a notation that looks like the letter U rotated 90 degrees to the right. Graphically, this can be represented as a circle contained within another, larger circle.

Some common sets in set theory include N, the set of all natural numbers; Z, the set of all integers; Q, the set of all rational numbers; R, the set of all real numbers; and C, the set of all complex numbers.

When two sets overlap but neither is completely embedded within the other, the whole thing is called a union of sets. This is represented by a symbol similar to the letter U, but slightly wider. In set notation, A U B means "the set of elements which are members of either A or B". Turn this symbol upside down, and you get the intersection of A and B, which refers to all elements which are members of both sets. In set theory sets can also be "subtracted" from each other, resulting in complements. For example, B - A is equivalent to the set of elements that are members of B but not A.

From the above foundations, most of mathematics is derived. Nearly all mathematical systems contain properties that can be described fundamentally in terms of set theory.


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Post 4


If all we can prove is that nothing is truly provable, and basic mathematical set theory is the basis of that proof, then where do we have to go besides insanity? Faith?

Post 3


A subsequent mathematician who chose to think on the edge of human thought was Kurt Godel, who established his first incompleteness theorem based on principles established in the infinite set theorem of Cantor. Godel's theorem effectively shows us that there are limitations to math and that certain mathematical principles simply need to be taken as true, though they are unprovable.

Post 2


Cantor even established an "infinity of infinities," going above and beyond what the human mind can fully comprehend. Understanding why this set behaves the way it seems to was an impossible task, which continued to drive mathematicians and cosmologists insane even after the death of Cantor.

Post 1

Georg Cantor established mathematical sets which were so abstract that the academic community made fun of him. Turns out he was way ahead of his time. His sets were made to describe infinity and supersets of infinitude. He effectively pushed the line and established the basis for modern theory of relativity. Unfortunately, his constant thinking outside the box paired with the unrelenting mockery from the mathematical community had a negative effect on the mental health of this genius.

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