筛选法 求素数 一个数的所有因素和

/*
---------------------------------------
    素数筛选法
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*/

#include<stdio.h>
int x[20000000];
int main()
{

    int temp, n, i;
    for( i = 0; i < 20000000; i ++ )
        x[i] = 0;
    x[i] = x[1] = 1;
    for( i = 2; i < 20000000; i ++ )
    {
        if( x[i] == 0 )
        {
            temp = 2 * i;
            while( temp <= 20000000)
            {
                x[temp] = 1;
                temp += i;
            }
        }
    }
    n = 19880100;
    while( n ++ && n < 19891235 )
    {
        if( x[n] == 0  && n % 10000 < 1235)
            printf("%d\n", n);

    }
    return 0;
}

/*
-------------------------------------------------
    筛选法求一个数因数的和（如12：1，2，3，4，6）
-------------------------------------------------
*/


#include<stdio.h>
#include<string.h>

long long num[500005];

void shaixuan( )
{
     for( int i = 1; i < 500005; ++i )
          num[i] = 1;
     for( int i = 2; i < 500005; ++i )
          for( int j = 2; j * i < 500005; ++j )
               num[i*j] += i;
 }
int main( )
{
    shaixuan( );
    int t,n;
    scanf( "%d",&t );
    while( t-- )
    {
           scanf( "%d",&n );
           printf( "%I64d\n",num[n] );
    }
    return 0;
}