MATH 21-127: Concepts of Mathematics
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In mathematics, a rational number is 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. Since q
may be equal to 1, every integer is a rational number. The set of all rational numbers,
often referred to as ”the rationals”, is usually denoted by a boldface Q (or blackboard
bold , Unicode ); it was thus denoted in 1895 by Giuseppe Peano after quoziente, Italian
for ”quotient”. The decimal expansion of a rational number always either terminates
after a finite number of digits or begins to repeat the same finite sequence of digits over
and over. Moreover, any repeating or terminating decimal represents a rational number.
These statements hold true not just for base 10, but also for any other integer base (e.g.
binary, hexadecimal). A real number that is not rational is called irrational. Irrational
numbers include √2, , e, and . The decimal expansion of an irrational number continues
without repeating. Since the set of rational numbers is countable, and the set of real
numbers is uncountable, almost allreal numbers are irrational.