Set Notation and Venn Diagrams

Number

Any well-defined group of things forms a set, and each thing belonging to it is an element of that set
Elements can be anything: numbers, letters, or coordinate pairs
Writing the elements inside curly brackets defines a set directly
- The factors of 12 form the set {1, 2, 3, 4, 6, 12}
A rule can replace the list, written with a colon read as "such that"
- {x : x is an integer, 1 ⩽ x ⩽ 5} is another way of writing {1, 2, 3, 4, 5}
- {(x, y) : y = mx + c} is the set of points lying on that line
Capital letters name sets and lower-case letters name elements

The universal set holds everything under consideration in that question, and this specification writes it as ℰ, a script capital E
Every other set in the question is contained within it
Other material uses the Greek letter xi, ξ, or the letter U for the same thing, and the diagram below is labelled that way, but ℰ is the symbol the syllabus lists and the one printed on these papers