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Euclid: Elementa

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Click to Expand/Collapse OptionTitle
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PROPOSITION 20. 
 
 
Prime numbers are more than any assigned multitude of prime numbers. 
 
 
Let A, B, C be the assigned prime numbers;  I say that there are more prime numbers than A, B, C. 
   
   
For let the least number measured by A, B, C be taken, and let it be DE;  Let the unit DF be added to DE.  Then EF is either prime or not.  First, let it be prime;  then the prime numbers A, B, C, EF have been found which are more than A, B, C. 
         
         
Next, let EF not be prime;  therefore it is measured by some prime number. [VII. 31]  Let it be measured by the prime number G.  I say that G is not the same with any of the numbers A, B, C. 
       
       
For, if possible, let it be so.  Now A, B, C measure DE;  therefore G also will measure DE.  But it also measures EF.  Therefore G, being a number, will measure the remainder, the unit DF: which is absurd.  Therefore G is not the same with any one of the numbers A, B, C.  And by hypothesis it is prime.  Therefore the prime numbers A, B, C, G have been found which are more than the assigned multitude of A, B, C.  Q. E. D. 
                 
                 
 
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