Unitary divisor

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A number $a$ is a unitary divisor of a number $b$ if $a$ and $\frac{b}{a}$ have only one common factor (1). Thus, 5 is a unitary divisor of 60, because 5 and $\frac{60}{5}=12$ have only one as a common factor, while 6 is a divisor but not a unitary divisor of 60, as 6 and $\frac{60}{6}=10$ have common factors other than 1 (such as 2).