Acetylcholine is an organic chemical that functions in the brain and body of many types of animals, including humans, as a neurotransmitterâa chemical released by nerve cells to send signals to other cells. Its name is derived from its chemical structure: it is an ester of acetic acid and choline. Parts in the body that use or are affected by acetylcholine are referred to as cholinergic. Substances that interfere with acetylcholine activity are called anticholinergics. Acetylcholine is the ...
Every interior algebra can be represented as a topological field of sets with its interior and closure operators corresponding to those of the topological space. Given a topological space the clopen sets trivially form a topological field of sets as each clopen set is its own interior and closure.
In the area of mathematics called set theory, a special case of the von Neumann ordinal construction defines the natural numbers as follows: 1  Set 0 = { }, the empty set, 2  Define S(a) = a  3  {a} for every set a. 4  By the axiom of infinity, there exists a set which contains 0 and is closed under the successor function.