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The Elements of Style (1918), by William Strunk, Jr. and E. B. White, is a prescriptive American English writing style guide comprising eight "elementary rules of usage", ten "elementary principles of composition", "a few matters of form", a list of forty-nine "words and expressions commonly misused", and a list of fifty-seven "words often misspelled". In 2011, Time magazine listed The Elements of Style as one of the 100 best and most influential books written in English since 1923.[1]
William Strunk, Jr. (1 July 1869 – 26 September 1946), was a professor of English at Cornell University and author of the The Elements of Style (1918), which, after being revised and enlarged by his former student E. B. White, became a highly influential guide to English usage during the late 20th century.
Strunk first taught mathematics at Rose Polytechnical Institute in Terre Haute, Indiana in 1890–91.[3] He then taught English at Cornell for 46 years
A set is a group of objects, numbers, and so forth. {1,2,3} is a set consisting of the numbers 1,2, and 3. Verbally, "3 is an element of the set {1,2,3}.” To show this symbolically, use the symbol ∈, which is read as "is an element of” or "is a member of.” Therefore, you could have written:
A subset is a set contained within another set, or it can be the entire set itself. The set {1,2} is a subset of the set {1,2,3}, and the set {1,2,3} is a subset of the set {1,2,3}.
In set theory, the union (denoted by ∪) of a collection of sets is the set of all distinct elements in the collection.[1] It is one of the fundamental operations through which sets can be combined and related to each other.
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The most general notion is the union of an arbitrary collection of sets, sometimes called an infinitary union. If M is a set whose elements are themselves sets, then x is an element of the union of M if and only if there is at least one element A of M such that x is an element of A. In symbols:
That this union of M is a set no matter how large a set M itself might be, is the content of the axiom of union in axiomatic set theory.
This idea subsumes the preceding sections, in that (for example) A ∪ B ∪ C is the union of the collection {A,B,C}. Also, if M is the empty collection, then the union of M is the empty set. The analogy between finite unions and logical disjunction extends to one between arbitrary unions and existential quantification.
The notation for the general concept can vary considerably. For a finite union of sets 
one often writes 
. Various common notations for
arbitrary unions include 
,
