Sets

A set is a collection of unique (no duplicates) objects, physical or abstract, called elements or members.
{a, b, c} No order, so same set as {c, a, b}
finite: {9, 3, -2, π}
infinite: {0,1,2,3...}
empty set: {} = ∅

interval: [a,b] all numbers between a and b, inclusive: a≤x≤b
interval: [a,b) all numbers between a and b. includes a but not b: a≤x<b
Ex.   (2,5]    "sandwiched" between endpoints 2 and 5

interval: [b,∞)    x≥b
interval: (-∞,b)    x<b
Complement of an interval is all the other numbers of R
Ex.   complement of (2,5] is (-∞,2]∪(5,∞)

Set operations:
union of two sets: elements that are in either set. OR
Ex.   {1,2,3} ∪ {2,3,4,5} = {1,2,3,4,5}
intersection of two sets: elements that are in both sets. AND
Ex.   {1,2,3} ∩ {2,3,4,5} = {2,3}