poj2478-Farey Sequence

http://poj.org/problem?id=2478

素数筛法+欧拉函数

 

#include<iostream>

#include<cstdio>

#include<cstring>



using namespace std; 



#define maxn  1000005 

#define INT __int64 

int Prime[ maxn + 1 ] ;

INT ans[ maxn + 1 ] ;



void prime()

{

	memset( Prime , 0 , sizeof( Prime ) ) ;

	Prime[ 0 ] = Prime[ 1 ] = 1 ;

	for( int i = 2 ; i * i <= maxn ; ++i )

	{

		if( !Prime[ i ] )

		{

			for( int j = i * i ; j <= maxn ; j += i )

				Prime[  j ] = 1 ;

		}

	}

}



void enlerfun()

{

	for( int i = 1 ; i <= maxn ; ++i )

		ans[ i ] = i ;

	for( int i = 2 ; i <= maxn ; ++i )

	{

		if( !Prime[ i ] )

		for( int j = i ; j <= maxn ; j += i )

		{

			ans[ j ] = ans[ j ] / i * ( i - 1 ) ;

		}

	}

}

	

int main()

{

	int n ;

	prime() ;

	enlerfun() ;

	for( int i = 3 ; i <= maxn ; ++i )

		ans[ i ] += ans[ i - 1 ] ;

	while( ~scanf( "%d" , &n ) && n )

	{

		printf( "%I64d\n" , ans[ n ] ) ;

	}

	return 0 ;

}