Rational number ($\mathbb{Q}$, $\frac{1}{2}$, $\frac{4}{3}$)

A "reduced fraction" is a fraction that has the smallest possible integer numerator and denominator for its value.

For example: is not a reduced fraction, because there is another fraction equal to it but with smaller numerator and denominator:

en.wikipedia.org/w/index.php?title=Farey_sequence&oldid=1223364653#Sequence_length_and_index_of_a_fraction where $\Phi (n)$ is the totient summatory function.