ACM-ICPC 2018 南京賽區網路預賽 J. Sum
A square-free integer is an integer which is indivisible by any square number except 11. For example, 6 = 2 \cdot 36=2⋅3 is square-free, but 12 = 2^2 \cdot 312=22⋅3 is not, because 2^222 is a square number. Some integers could be decomposed into product of two square-free integers, there may be more than one decomposition ways. For example, 6 = 1\cdot 6=6 \cdot 1=2\cdot 3=3\cdot 2, n=ab6=1⋅6=6⋅1=2⋅3=3⋅2,n=ab and n=ban=ba are considered different if a \not = ba̸=b. f(n)f(n) is the number of decomposition ways that n=abn=ab such that aa and bb are square-free integers. The problem is calculating \sum_{i = 1}^nf(i)∑i=1nf(i).
Input
The first line contains an integer T(T\le 20)T(T≤20), denoting the number of test cases.
For each test case, there first line has a integer n(n \le 2\cdot 10^7)n(n≤2⋅107).
Output
For each test case, print the answer \sum_{i = 1}^n f(i)∑i=1nf(i).
Hint
\sum_{i = 1}^8 f(i)=f(1)+ \cdots +f(8)∑i=18f(i)=f(1)+⋯+f(8) =1+2+2+1+2+4+2+0=14=1+2+2+1+2+4+2+0=14.
樣例輸入複製
2 5 8
樣例輸出複製
8 14
題目來源
思路: 相同因子個數不超過2的數稱為squarefree(SF),F【i】= A*B的種數,AB都是SF,AB不相等的時候AB 和BA算兩種方案,求1~n內F【i】字首和。
魔改線性篩,太6了,Orz..................
程式碼:
#include<bits/stdc++.h> using namespace std; const int maxn=2e7+5; long long ans[maxn]; int tot,prime[maxn],vis[maxn],f[maxn];///prime儲存遍歷到的素數 void doit(){ memset(vis,0,sizeof(vis)); memset(prime,0,sizeof(prime)); memset(f,0,sizeof(f)); prime[1]=1; ans[1]=1; tot=0; for(int i=2;i<maxn;i++){ if(!vis[i]){ prime[tot++]=i; f[i]=2; } for(int j=0;j<tot&&i*prime[j]<maxn;j++){ vis[i*prime[j]]=1; if(i%prime[j]==0){ if(i%(prime[j]*prime[j])==0) f[i*prime[j]]=0;///大於等於三次方GG else f[i*prime[j]]=f[i]/2;///等於二次方得變一半,相當在原基礎上少了另一個素數 break; } else f[prime[j]*i]=f[i]*2; } ans[i]=ans[i-1]+f[i]; } } int main(){ doit(); int t; scanf("%d",&t); while(t--){ int n; scanf("%d",&n); printf("%lld\n",ans[n]); } }