題解

話說LOJ說我今天宜學數論= =看到小迪學了杜教篩去蹭了一波小迪做的題

標解的杜教篩的函數不懂啊，怎麽推的毫無思路= =
所以寫了個復雜度稍微高一點的？？

首先，我們發現f是個積性函數，那麽我們就有……
\(\prod_{i = 1}^{k}f(p_{i}^{a_{i}})\)
我們發現，對於每個質因子，gcd是取較小值，lcm取較大值
\(f(lcm(x,y)) * f(gcd(x,y)) = \prod_{i = 1}^{k} f(p_{i}^{max(a_{i},b_{i}) + min(a_{i},b_{i})})\)
\(max(a,b) + min(a,b) = a + b\)

所以我們只要求出\([\sum_{i = 1}^{n} f(i)]^2\)就是答案了！

怎麽求呢
\(S(n) = \sum_{i = 1}^{n}\sum_{d | i} \mu(d)\cdot d\)
\(S(n) = \sum_{d = 1}^{n}\sum_{d | i} \mu(d)\cdot d\)
\(S(n) = \sum_{d = 1}^{n} \mu(d)\cdot d \cdot \lfloor \frac{n}{d} \rfloor\)

\(\sum_{i = 1}^{n} [i = 1] = \sum_{i = 1}^{n} \sum_{d |i} \mu(d) \cdot d \cdot \frac{i}{d} = \sum_{k = 1}^{n} k \sum_{d}^{\frac{n}{k}} \mu(d) \cdot d\)
所以最後就是
\(S(n) = \frac{n(n + 1)(2n + 1)}{6} - \sum_{i = 2}^{n} S(\lfloor \frac{n}{i}\rfloor)\)

代碼

#include <iostream>
#include <cstdio>
#include <vector>
#include <algorithm>
#include <cmath>
#include <cstring>
#include <map>
//#define ivorysi
#define pb push_back
#define space putchar(‘ ‘)
#define enter putchar(‘\n‘)
#define mp make_pair
#define pb push_back
#define fi first
#define se second
#define mo 974711
#define MAXN 1000000
#define RG register
using namespace std;
typedef long long int64;
typedef double db;
template<class T>
void read(T &res) {
    res = 0;char c = getchar();T f = 1;
        while(c < ‘0‘ || c > ‘9‘) {
            if(c == ‘-‘) f = -1;
            c = getchar();
        }
        while(c >= ‘0‘ && c <= ‘9‘) {
        res = res * 10 + c - ‘0‘;
        c = getchar();
    }
    res *= f;
}
template<class T>
void out(T x) {
    if(x < 0) {putchar(‘-‘);x = -x;}
    if(x >= 10) {
        out(x / 10);
    }
    putchar(‘0‘ + x % 10);
}
int N;
const int MOD = 1000000007;
struct node {
    int x,v,next;
}E[2000006];
int head[mo + 5],sumE;
int prime[MAXN + 5],tot,S[MAXN + 5],mu[MAXN + 5];
bool nonprime[MAXN + 5];
int inc(int a,int b) {
    a = a + b;
    if(a >= MOD) a -= MOD;
    return a;
}
void add(int u,int x,int v) {
    E[++sumE].x = x;E[sumE].v = v;E[sumE].next = head[u];
    head[u] = sumE;
}
void Insert(int x,int v) {
    add(x % mo,x,v);
}
int Query(int x) {
    int u = x % mo;
    for(int i = head[u] ; i ; i = E[i].next) {
        if(E[i].x == x) return E[i].v;
    }
    return -1;
}
int f(int x) {
    if(x <= MAXN) return S[x];
    int c = Query(x);
    if(c != -1) return c;
    int res = 0;
    for(int i = 2 ; i <= x ; ++i) {
        int r = x / (x / i);
        res = inc(res,1LL * (r - i + 1) * (r + i) / 2 % MOD * f(x / i) % MOD);
        i = r;
    }
    res = inc(1,MOD - res);
    Insert(x,res);
    return res;
}
void Solve() {
    mu[1] = 1;S[1] = 1;
    for(int i = 2 ; i <= MAXN ; ++i) {
        if(!nonprime[i]) {
            mu[i] = -1;
            prime[++tot] = i;
        }
        for(int j = 1 ; j <= tot ; ++j) {
            if(prime[j] > MAXN / i) break;
            nonprime[i * prime[j]] = 1;
            if(i % prime[j] == 0) break;
            else mu[i * prime[j]] = -mu[i];
        }
        S[i] = (S[i - 1] + mu[i] * i + MOD) % MOD;
    }
    read(N);
    int res = 0;
    for(int i = 1 ; i <= N ; ++i) {
        int r = N / (N / i);
        res = inc(1LL * (f(r) + MOD - f(i - 1)) * (N / i) % MOD,res);
        i = r;
    }
    out(1LL * res * res % MOD);enter;
}
int main() {
#ifdef ivorysi
    freopen("f1.in","r",stdin);
#endif
    Solve();
    return 0;
}

【51nod】2026 Gcd and Lcm