- Wasat (stars.astro.illinois.edu)
WASAT (Delta Geminorum). With a name that does not ring much of a bell among astronomers, and shining only at the bright end of fourth magnitude, neither does the star itself.
- Empty set (Wikipedia)
In mathematics, the empty set is the unique set having no elements; its size or cardinality (count of elements in a set) is zero. Some axiomatic set theories ensure that the empty set exists by including an axiom of empty set, while in other theories, its existence can be deduced. Many possible properties of sets are vacuously true for the empty set.