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Euclid: Elementa

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PROPOSITION 27. 
 
 
If two numbers be prime to one another, and each by multiplying itself make a certain number, the products will be prime to one another; and, if the original numbers by multiplying the products make certain numbers, the latter will also be prime to one another [and this is always the case with the extremes]. 
 
 
Let A, B be two numbers prime to one another, let A by multiplying itself make C, and by multiplying C make D, and let B by multiplying itself make E, and by multiplying E make F;  I say that both C, E and D, F are prime to one another. 
   
   
For, since A, B are prime to one another,  and A by multiplying itself has made C,  therefore C, B are prime to one another. [VII. 25]  Since then C, B are prime to one another, and B by multiplying itself has made E,  therefore C, E are prime to one another. [id.]  Again, since A, B are prime to one another, and B by multiplying itself has made E,  therefore A, E are prime to one another. [id.]  Since then the two numbers A, C are prime to the two numbers B, E, both to each,  therefore also the product of A, C is prime to the product of B, E. [VII. 26]  And the product of A, C is D, and the product of B, E is F.  Therefore D, F are prime to one another.  Q. E. D. 
                       
                       
 
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