hduoj-2824【欧拉函数】

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The Euler function

Time Limit: 2000/1000 MS (Java/Others)    Memory Limit: 32768/32768 K (Java/Others)
Total Submission(s): 6405    Accepted Submission(s): 2691

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

思路：欧拉函数裸题，注意优化内存，开一个数组就行。

#include<cstdio>#include<algorithm>#include<cstring>#define LL long longusing namespace std;const int MAXN=3e6;int a,b;LL eu[MAXN+10]={0};void euler(){eu[1]=1;for(int i=2;i<=MAXN;i++){if(!eu[i]){for(int j=i;j<=MAXN;j+=i){if(!eu[j])eu[j]=j;eu[j]=eu[j]/i*(i-1);}}}for(int i=1;i<=MAXN;i++)eu[i]=eu[i-1]+eu[i];}int main(){euler();while(~scanf("%d%d",&a,&b)){printf("%lld\n",eu[b]-eu[a-1]);}return 0;}