HDU 2824(欧拉函数)

The Euler function

Time Limit: 2000/1000 MS (Java/Others)    Memory Limit: 32768/32768 K (Java/Others)

链接：2824 

Problem Description

The Euler function phi is an important kind of function in number theory, (n) represents the amount of the numbers which are smaller than n and coprime to n, and this function has a lot of beautiful characteristics. Here comes a very easy question: suppose you are given a, b, try to calculate (a)+ (a+1)+....+ (b)

 

Input

There are several test cases. Each line has two integers a, b (2<a<b<3000000).

 

Output

Output the result of (a)+ (a+1)+....+ (b)

 

Sample Input

3 100

 

Sample Output

3042

 

Source

2009 Multi-University Training Contest 1 - Host by TJU

 

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代码：

#include<iostream>
using namespace std;
bool flag[3000005];
int phi[3000005]={0,1},p[3000005]={0},cnt=0;
int main()
{
    for(int i=0;i<=3000004;i++)flag[i]=1;
    for(int i=2;i<3000000;i++)
    {
        if(flag[i])
        {
            p[cnt++]=i;
            phi[i]=i-1;
        }
        for(int j=0;j<cnt;j++)
        {
            if(i*p[j]>3000000)break;
            flag[i*p[j]]=0;
            if(i%p[j]==0)
            {
                phi[i*p[j]]=p[j]*phi[i];
                break;
            }
            else phi[i*p[j]]=(p[j]-1)*phi[i];
        }
    }
    int i,x,y;
    while(cin>>x>>y)
    {
    long long ans=0;
    for(i=x;i<=y;i++)ans+=phi[i];
    cout<<ans<<endl;
}
    return 0;
}