
INTEGERS = {..., -3, -2, -1, 0, 1, 2, 3, ...}
WHOLE NUMBERS = {0, 1, 2, 3, 4, 5, ...}
RATIONAL NUMBERS: any number that can be expressed as the quotient or fraction p/q of two integers, a numerator p and a non-zero denominator q.
IRRATIONAL NUMBERS: all real numbers which are not rational numbers.