The backslash "\" is the symbol of the set operation. The relative difference set of sets U and A, with the symbol U \ A, is all the elements in set U but not in set A. The relative difference set {1, 2,3} \ {2,3,4} is {1}, but the relative difference set.
When set A is a subset of set U, relative difference set u \ a is also called the complement set of set A in set U. If we study venn diagram, when set U is a complete set and the definition of complete set can be found from the context, we will use a instead of u \ a. ..
Representation of other sets:
R+: positive real number set
R-: negative real number set
C: complex set
: empty set (a set without any elements)
N* or N+: positive integer set {1, 2,3, …}
Q+: Set of Positive Rational Numbers
Q-: set of negative rational numbers