[category theory] In category theory, a small set is one in a fixed universe of sets (as the word universe is used in mathematics in general). Thus, the category of small sets is the category of all sets one cares to consider. This is used when one does not wish to bother with set-theoretic concerns of what is and what is not considered a s...
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http://en.wikipedia.org/wiki/Small_set_(category_theory)
[combinatorics] ==Open problems== It is not known how to identify whether a given set is large or small in general. As a result, there are many sets which are not known to be either large or small. Paul Erdős famously asked the question of whether any set that does not contain arbitrarily long arithmetic progressions must necessarily be sm...
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http://en.wikipedia.org/wiki/Small_set_(combinatorics)
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