题目链接:https://vjudge.net/problem/POJ-2478 Farey Sequence Time Limit: 1000MS   Memory Limit: 65536K Total Submissions: 17753   Accepted: 7112 Description The Farey Sequence Fn for any integer n with n >= 2 is the set of irreducible rational numbers a/b…

Farey Sequence 题意:给定一个数n,求在[1,n]这个范围内两两互质的数的个数.(转化为给定一个数n,比n小且与n互质的数的个数) 知识点: 欧拉函数: 普通求法: int Euler(int n) { int ans=n; for(int i=0;i<cnt&&prime[i]<=n;i++) { if(n%prime[i]==0) { ans=ans-ans/prime[i]; while(n%prime[i]==0) n/=prime[i]; } } if(…

仔细看看题目,按照题目要求 其实就是 求 小于等于n的 每一个数的 欧拉函数值  的总和,为什么呢,因为要构成 a/b 然后不能约分  所以 gcd(a,b)==1,所以  分母 b的 欧拉函数值  就是 以b为分母的 这样的数有几个,分母b的范围 是小于等于n,所以 直接套一个模版就可以了 ,网上找的  说筛选的比较好,下面代码中有一个 注释掉的 模版 貌似 是错的,还不清楚为什么  弄清楚了 重新 注明一下  #include<iostream> #include<cstdio>…

hdu1787,直接求欧拉函数 #include <iostream> #include <cstdio> using namespace std; int n; int phi(int n){ int ans=n; for(int i=2; i*i<=n; i++) if(n%i==0){ ans -= ans / i; while(n%i==0) n /= i; } if(n>1) ans -= ans / n; return ans; } int main(){…

http://poj.org/problem?id=2478 求欧拉函数的模板. 初涉欧拉函数,先学一学它主要的性质. 1.欧拉函数是求小于n且和n互质(包含1)的正整数的个数. 记为φ(n). 2.欧拉定理:若a与n互质.那么有a^φ(n) ≡ 1(mod n),经经常使用于求幂的模. 3.若p是一个质数,那么φ(p) = p-1.注意φ(1) = 1. 4.欧拉函数是积性函数: 若m与n互质,那么φ(nm) = φ(n) * φ(m). 若n = p^k且p为质数,那么φ(n) = p^k…

洛谷传送门 Farey Sequence (格式太难调,题面就不放了) 分析: 实际上求分数个数就是个幌子,观察可以得到,所求的就是$\sum^n_{i=2}\phi (i)$,所以直接欧拉筛+前缀和即可. Code: #include<cstdio> #include<cstring> #include<cstdlib> #include<cmath> #include<iostream> #include<iomanip> #in…

Farey Sequence Time Limit: 1000MS   Memory Limit: 65536K       Description The Farey Sequence Fn for any integer n with n >= 2 is the set of irreducible rational numbers a/b with 0 < a < b <= n and gcd(a,b) = 1 arranged in increasing order. Th…

链接: https://vjudge.net/problem/POJ-2478 题意: The Farey Sequence Fn for any integer n with n >= 2 is the set of irreducible rational numbers a/b with 0 < a < b <= n and gcd(a,b) = 1 arranged in increasing order. The first few are F2 = {1/2} F3 =…

Farey Sequence Time Limit: 1000MS   Memory Limit: 65536K Total Submissions: 14291   Accepted: 5647 Description The Farey Sequence Fn for any integer n with n >= 2 is the set of irreducible rational numbers a/b with 0 < a < b <= n and gcd(a,b)…

题目大意 直接看原文吧.... The Farey Sequence Fn for any integer n with n >= 2 is the set of irreducible rational numbers a/b with 0 < a < b <= n and gcd(a,b) = 1 arranged in increasing order. The first few areF2 = {1/2}F3 = {1/3, 1/2, 2/3}F4 = {1/4, 1/3…