These are all expressed in capital letters, mainly including
R: real number set
Q: Rational number set
Z: integer set
N: natural number set
Add "+"after these letters to indicate the masculine part.
N+: Positive natural number set is a set of positive integers.
Z+: positive integer set
R+: positive real number set
Adding "*" to the right of the letter indicates the part other than 0.
N*: natural number set other than 0 is a set of positive integers.
Z*: non-zero integer set
R*: non-zero real number set
Sets are usually represented by uppercase letters A, B, C ... Elements are usually represented by lowercase letters A, B, C. ...