## Construction of the Rationals

Rational numbers can be constructed from the integers and integer arithmetic. A rational number is constructed from an ordered pair of integers in a fraction: (= A (</> (top A) (bottom A))). The first number in the ordered pair: (top A) is the top or numerator. The second number (bottom A) in the pair is the bottom or denominator.

Two rational numbers A and B are equal if the ratio between them is equal (= (</> (top A) (bottom A)) (</> (top B) (bottom B))), but this equation requires division which has not been properly defined yet. This equation is equivalent to (= (* (top A) (bottom B)) (* (top B) (bottom A))). However, division by zero adds a further complication. If we used the equation (= (* (top A) (bottom B)) (* (top B) (bottom A))) to define rational equality, then the fraction (</> (0) (0)) would be equal to all numbers. So we add a further restriction that rational numbers are only equal if both denominators are not (0) or both are (0). The full definition of rational equality is given here.

There is more than one fraction that is equal to a given rational number. For example, one half can be represented as (</> (1) (2)), or (</> (2) (4)) or (</> (5) (10)). Each rational number represents a class of equivalent ordered pairs.

The difficult part of constructing the rationals is to shown that the arithmetic operations are properly defined over these equivalence classes. Rational multiplication is defined by multiply the numerators and denominators (= (* (</> a b) (</> c d)) (</> (z.* a c) (z.* b d))) /edit/peano/rationals.gh/qmulfrac. Rational addition is defined by the formula (= (+ (</> a b) (</> c d)) (</> (z.+ (z.* a d) (z.* c b)) (z.* b d))) /edit/peano/rationals.gh/qaddfrac. We can prove all the important properties of addition and multiplication with these definitions: