Some sets are commonly used

**
N
**
: the set of all
natural numbers

**
Z
**
: the set of all
integers

**
Q
**
: the set of all
rational numbers

**
R
**
: the set of
real numbers

**
Z+
**
: the set of
positive integers

**
Q+
**
: the set of positive
rational numbers

**
R+
**
: the set of
positive real numbers