poj2478

欧拉函数，n每扩大一个，就会有euler(n)个数字加入到集合中，所以欧拉函数的前n项和即为所求。

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      #include 
    
      <
    
      iostream
    
      >
    
      
#include 
    
      <
    
      cstdio
    
      >
    
      
#include 
    
      <
    
      cstdlib
    
      >
    
      
#include 
    
      <
    
      cstring
    
      >
    
      

    
      using
    
       
    
      namespace
    
       std;


    
      #define
    
       maxn 1000006
    
      


    
      long
    
       
    
      long
    
       phi[maxn], num[maxn];

    
      int
    
       n;


    
      int
    
       main()
{
 
    
      //
    
      freopen("t.txt", "r", stdin);
    
      

    
       
    
      int
    
       i, j;
 
    
      for
    
       (i 
    
      =
    
       
    
      1
    
      ; i 
    
      <=
    
       maxn; i
    
      ++
    
      )
 phi[i] 
    
      =
    
       i;
 
    
      for
    
       (i 
    
      =
    
       
    
      2
    
      ; i 
    
      <=
    
       maxn; i 
    
      +=
    
       
    
      2
    
      )
 phi[i] 
    
      /=
    
       
    
      2
    
      ;
 
    
      for
    
       (i 
    
      =
    
       
    
      3
    
      ; i 
    
      <=
    
       maxn; i 
    
      +=
    
       
    
      2
    
      )
 
    
      if
    
       (phi[i] 
    
      ==
    
       i)
 {
 
    
      for
    
       (j 
    
      =
    
       i; j 
    
      <=
    
       maxn; j 
    
      +=
    
       i)
 phi[j] 
    
      =
    
       phi[j] 
    
      /
    
       i 
    
      *
    
       (i 
    
      -
    
       
    
      1
    
      );
 }
 num[
    
      0
    
      ] 
    
      =
    
       
    
      0
    
      ;
 
    
      for
    
       (
    
      int
    
       i 
    
      =
    
       
    
      1
    
      ; i 
    
      <
    
       maxn; i
    
      ++
    
      )
 num[i] 
    
      =
    
       num[i 
    
      -
    
       
    
      1
    
      ] 
    
      +
    
       phi[i];
 
    
      while
    
       (scanf(
    
      "
    
      %d
    
      "
    
      , 
    
      &
    
      n), n 
    
      !=
    
       
    
      0
    
      )
 {
 printf(
    
      "
    
      %lld\n
    
      "
    
      , num[n] 
    
      -
    
       
    
      1
    
      );
 }
 
    
      return
    
       
    
      0
    
      ;
}