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          Abstract: | 
           
              Let   be the  -th positive integer number which can be written as a power  ,  , of a prime   (  is fixed). Let    denote the number of prime powers  ,  , not exceeding  . We study the asymptotic behaviour of the sequence   and of the function  . We prove that the sequence   has an asymptotic expansion comparable to that of   (the Cipolla's expansion). ;
             
          
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