Two numbers are relatively prime if they do not share any factors, other than 1. Is it possible for fractions to be relatively prime? To reword this, do fractions even have factors?
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2One can extend gcd and lcm to rationals. – Bill Dubuque May 18 '14 at 00:44
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Relatively prime applies usually to integers, but like Bill Dubuque's link says you can extend the idea of factors to fractions by having a common base.
$\frac{1}{3}, \frac{2}{5}$ become $\frac{5}{15}, \frac{6}{15}$ which have relatively prime numerators. In this sense fractions are just integers split up into a common number of parts.
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Every rational number is of the form $\prod_{n=1}^\infty p_n^{e_n}$ where $p_n$ is the $n$th prime number and each $e_n$ is some integer. The difference between that statement and a similar one about integers is that with integers one has $e_n\ge0$ for every value of $n$. Two integers are relatively prime if the set of primes with non-zero exponents for one of them is disjoint from the set of primes with nonzero exponents for the other. One could use that same definition with rational numbers and say, for example, that $8/35$ and $11/9$ are relatively prime because $$ \frac{8}{35} = 2^3 5^{-1} 7^{-1} \text{ and }\frac{11}{9} = 3^{-2} 11^1 $$ and $$ \{2,5,7\} \cap \{3,11\}=\varnothing. $$
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1hi sir, i did not understand something in your explanation.We know that if two number is relatively prime ,then GCD of them are equal to 1.However, gcd of $(8/35),(11/9)$ is $(1/315)$ ,so it is not equal 1.If you have leisure time ,can you enlighten me please? – Sep 18 '20 at 20:26
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@leonard : Possibly the answer is that the proposition that relative primality implies the gcd is $1$ needs a hypothesis saying it applies only to integers. I may come back to this question this weekend. – Michael Hardy Sep 18 '20 at 21:24
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by the way, https://math.stackexchange.com/questions/3831695/determining-whether-or-not-given-fractions-are-relatively-primes – Sep 19 '20 at 04:00