ACM-ICPC 2018南京賽區網路預賽 J Sum(線性篩)
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
題意:f[i]表示i可以分解成為兩個不能被平方數整除的數的個數(順序不同算不同),對f[i]求字首和。
思路:對於n,可以表示為n = a1^p1*a2^p2*...*an^pn,如果有一個pi>2,那麼f[n] = 0,否則f[n] = 2^(sum(pi==1))。可以線性篩篩出來。
#include<bits/stdc++.h> using namespace std; typedef long long ll; const int maxn = 2e7+10; bool check[maxn]; int prime[maxn]; ll f[maxn]; int tot=0; int main(){ check[1] = 1; f[1] = 1; int num; for(int i=2;i<maxn;i++){ if(check[i]==0){ //prime prime[tot++]=i; f[i] = 2; } for(int j = 0;j<tot&&(ll)i*prime[j]<maxn;++j) { num = i*prime[j]; check[num] = 1; if(i%prime[j]) { f[num] = f[i]*2; } else if((ll)i%(prime[j]*prime[j])==0) { f[num] = 0; } else { f[num] = f[num/prime[j]/prime[j]]; break; } } } for(int i = 2;i<maxn;i++) f[i] += f[i-1]; int t; int n; scanf("%d",&t); while(t--) { scanf("%d",&n); printf("%lld\n",f[n]); } }