Pairs Forming LCM(素因子分解)

http://acm.hust.edu.cn/vjudge/contest/view.action?cid=109329#problem/B    全题在文末。

^{题意：在a,b中(a,b<=n)(1 ≤ n ≤ 1014),有多少组(a,b)  (a<b)满足lcm(a,b)==n;}

 

先来看个知识点：

素因子分解：n = p1 ^ e1 * p2 ^ e2 *..........*pn ^ en

for i in range(1,n):

        ei 从0取到ei的所有组合

必能包含所有n的因子。

现在取n的两个因子a,b

a=p1 ^ a1 * p2 ^ a2 *..........*pn ^ an

b=p1 ^ b1 * p2 ^ b2 *..........*pn ^ bn

gcd(a,b)=p1 ^ min(a1,b1) * p2 ^ min(a2,b2) *..........*pn ^ min(an,bn)

lcm(a,b)=p1 ^ max(a1,b1) * p2 ^ max(a2,b2) *..........*pn ^ max(an,bn)

哈哈，又多了种求gcd,lcm的方法。

 

题解：

先对n素因子分解，n = p1 ^ e1 * p2 ^ e2 *..........*pk ^ ek，

lcm(a,b)=p1 ^ max(a1,b1) * p2 ^ max(a2,b2) *..........*pk ^ max(ak,bk)

所以，当lcm(a,b)==n时，max(a1,b1)==e1,max(a2,b2)==e2,…max(ak,bk)==ek

当ai == ei时，bi可取 [0, ei] 中的所有数  有 ei+1 种情况，bi==ei时同理。

那么就有2(ei+1)种取法,但是当ai = bi = ei 时有重复，所以取法数为2(ei+1)-1=2*ei+1。
除了 (n, n) 所有的情况都出现了两次  那么满足a<=b的有 (2*ei + 1)) / 2 + 1 个

 

 

#include<iostream>
#include<cstdio>
#include<cstring>
using namespace std;
typedef long long LL;
const int N=1e7+5;
const int NN=1e6;
unsigned int prime[NN],cnt;           //prime[N]会MLE
bool vis[N];

void is_prime()
{
    cnt=0;
    memset(vis,0,sizeof(vis));
    for(int i=2;i<N;i++)
    {
        if(!vis[i])
        {
            prime[cnt++]=i;
            for(int j=i+i;j<N;j+=i)
            {
                vis[j]=1;
            }
        }
    }
}

int main()
{
    is_prime();
    int t;
    cin>>t;
    for(int kase=1;kase<=t;kase++)
    {
        LL n;
        cin>>n;
        int ans=1;
        for(int i=0;i<cnt&&prime[i]*prime[i]<=n;i++)
        {
            if(n%prime[i]==0)
            {
                int e=0;
                while(n%prime[i]==0)
                {
                    n/=prime[i];
                    e++;
                }
                ans*=(2*e+1);
            }
        }
        if(n>1)
            ans*=(2*1+1);
        printf("Case %d: %d\n",kase,(ans+1)/2);
    }
}

 

题目：

B - Pairs Forming LCM

Time Limit:2000MS     Memory Limit:32768KB     64bit IO Format:%lld & %llu

Submit Status

Description

Find the result of the following code:

long long pairsFormLCM( int n ) {
long long res = 0;
for( int i = 1; i <= n; i++ )
for( int j = i; j <= n; j++ )
if( lcm(i, j) == n ) res++; // lcm means least common multiple
return res;
}

A straight forward implementation of the code may time out. If you analyze the code, you will find that the code actually counts the number of pairs(i, j) for which lcm(i, j) = n and (i ≤ j).

Input

Input starts with an integer T (≤ 200), denoting the number of test cases.

Each case starts with a line containing an integer n (1 ≤ n ≤ 10¹⁴).

Output

For each case, print the case number and the value returned by the function 'pairsFormLCM(n)'.

Sample Input

15

2

3

4

6

8

10

12

15

18

20

21

24

25

27

29

Sample Output

Case 1: 2

Case 2: 2

Case 3: 3

Case 4: 5

Case 5: 4

Case 6: 5

Case 7: 8

Case 8: 5

Case 9: 8

Case 10: 8

Case 11: 5

Case 12: 11

Case 13: 3

Case 14: 4

Case 15: 2