N is a natural number set, also called a set of nonnegative integers, such as 0, 1, 2, 3. ......

N+ (or N*) is a set of positive integers, such as 1, 2,3. ......

Z is a set of all integers, such as -2,-1, 0, 1, 2. ......

Q is a set of rational numbers and r is a set of real numbers.