GYM103415K Magus Night

Solution
1012 Words

简要题意

对所有长度为 $n$ ，元素不超过 $m$ ，$\texttt{lcm} \ge p$，$\texttt{gcd} \le q$ 数列求积的和

分析

原题意可转化为全部 数列的贡献去掉$\texttt{lcm} < p$、$\texttt{gcd} > q$，再加上 $\texttt{lcm} < p$，$\texttt{gcd} > q$ 数列的贡献

第一部分我们考虑二（多）项式定理，故总和为$H(m)=(\sum_{i=1}^mi)^n=(\frac{m(m+1)}{2})^n$

第二部分我们考虑枚举 $\texttt{lcm}$ ，设 $g(x)$ 表示 $\texttt{lcm}=x$ 数列的贡献

则可以莫比乌斯反演，$g(x)=\sum_{d|x} \mu(d) h(\frac{x}{d})$， 其中 $h(x)$ 是所有 $\texttt{lcm}$ 为 $x$ 因数的数列的贡献

$h(x)$ 的表达式可以写成 $(\sum_{i|x}i)^n$ ，于是我们就可以愉快计算 $g(x)$ 了

第三部分同样考虑枚举 $\texttt{gcd}$，设 $G(x)$ 表示 $\texttt{gcd}=x$ 数列的贡献

考虑把 $x$ 除掉变成互质数列，互质数列贡献 $F(m)=\sum_{d=1}^x \mu(d) H(\frac{m}{d})$（好像这里可以整除分块？）

则 $G(x)=x^n F(\frac{m}{x})$ ，于是也可以快乐计算 $G(x)$ 了

第四部分考虑同时枚举 $\texttt{gcd}$ 和 $\texttt{lcm}$，由于 $\texttt{lcm} < p$，且 $\texttt{gcd}$ $\mid$ $\texttt{lcm}$，故这里复杂度是正确的

先考虑一个$\texttt{gcd} = 1, \texttt{lcm}=x$ 的数列，它的贡献 $f(x)=\sum_{d|x} \mu(d) g(\frac{x}{d})$

再考虑一个$\texttt{gcd}=t$ 和 $\texttt{lcm}=xt$ 的数列，则 $f(x)$ 只要乘上 $t^n$ 即可

综上所述，总贡献为

代码

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#include <bits/stdc++.h>
#define R register
#define ll long long
#define meow(cat...) fprintf(stderr, cat)

const ll MaxN = 2e5 + 10;
const ll mod = 998244353;
const ll inv2 = (mod + 1) / 2;

std::vector<ll> d[MaxN];
ll n, m, p, q, cnt, ans, f[MaxN], g[MaxN], h[MaxN], pw[MaxN], mu[MaxN];
ll vis[MaxN], s[MaxN], pr[MaxN], F[MaxN], G[MaxN], H[MaxN];

ll sum(ll a, ll b) { return ((a + b) % mod + mod) % mod; }
ll Add(ll &a, ll b) { return a = sum(a, b); }

ll fast_pow(ll a, ll b)
{
    ll res = 1;
    while(b)
    {
        if(b & 1)
            res = res * a % mod;
        a = a * a % mod, b >>= 1;
    }
    return res;
}

void init()
{
    mu[1] = 1;
    for(ll i = 2; i < MaxN; i++)  
    {
        if(!vis[i]) pr[++cnt] = i, mu[i] = -1;
        for(ll j = 1; j <= cnt && i * pr[j] < MaxN; j++)
        {
            vis[i * pr[j]] = 1;
            if(i * pr[j] == 0)
                { mu[i * pr[j]] = 0; break; }
            mu[i * pr[j]] = -mu[i];
        }
    }
    for(ll i = 1; i < MaxN; i++)
        s[i] = sum(s[i - 1], mu[i]), Add(s[i], mod);
}

inline ll read()
{
    ll x = 0;
    char ch = getchar();
    while(ch > '9' || ch < '0')
        ch = getchar();
    while(ch <= '9' && ch >= '0') 
        x = (x << 1) + (x << 3) + (ch ^ 48), ch = getchar();
    return x;
}

signed main()
{   
    n = read(), m = read(), p = read(), q = read(), init();
    for(ll i = 1; i <= m; i++)
        H[i] = i * (i + 1) % mod * inv2 % mod, H[i] = fast_pow(H[i], n); 
    for(ll i = 1; i <= m; i++)
        for(ll j = 1; j * j <= i; j++)
            if(i % j == 0)
            {
                d[i].push_back(j);
                if(j * j != i)
                    d[i].push_back(i / j);
            }
    // meow("1 %d\n", clock());
    for(ll i = 1; i <= m; i++)
    {
        pw[i] = fast_pow(i, n);
        for(ll j = 0; j < d[i].size(); j++)
            Add(h[i], d[i][j]);
        // printf("$ %lld %lld\n", i, h[i]);
        h[i] = fast_pow(h[i], n);
    }
    // meow("2 %d\n", clock());
    for (ll i = 1; i <= m; i++) 
    {
        g[i] = h[i];
        for (auto j : d[i]) 
        {
            if (j == i) continue;
            Add(g[i], mod - g[j]);
        }
    }
    // meow("3 %d\n", clock());
    // meow("4 %d\n", clock());
    // for(ll i = 1; i <= m; i++)
    // {
    //     printf("i: %d ", i);
    //     for(ll l = 1, r; l <= i; l = r + 1)
    //         r = i / (i / l), Add(F[i], sum(s[r], mod - s[l - 1]) * H[i / r] % mod), 
    //             printf("%d %d %d | ", l, r, i / l);
    //     puts("");
        
    // }
    for (int i = m; i >= 1; --i) 
    {
        F[i] = H[m / i] * pw[i] % mod;
        for (int j = i + i; j <= m; j += i) 
        Add(F[i], mod - F[j]);
    }
    // meow("5 %d\n", clock());
    for(ll i = 1; i <= m; i++)
        G[i] = (F[i] % mod + mod) % mod;
    ll res1 = 0, res2 = 0, res3 = 0;
    for(ll x = 1; x < p; x++) Add(res1, g[x]);
    // meow("6 %d\n", clock());
    for(ll t = q + 1; t <= m; t++) Add(res2, G[t]);
    // for(ll i = 1; i <= m; i++)
    //     for(ll j = 1; i * j <= m; j++)
    //         Add(f[i * j], g[i] * mu[j]);        
    // for(ll t = q + 1; t < p; t++)
    //     for(ll x = 1; x <= (p - 1) / t; x++)
    //         Add(res3, pw[t] * f[x] % mod);
    for (int i = 1; i <= m; i++)
        g[i] = sum(g[i], g[i - 1]);
    for (int i = q + 1; i < p; i++)
        f[i] = g[(p - 1) / i] * pw[i] % mod;
    for (int i = p - 1; i > q; i--)
    {
        for (int j = 2 * i; j < p; j += i)
            f[i] = sum(f[i], mod - f[j]);
        res3 = sum(res3, f[i]);
    }
    // meow("7 %d\n", clock());
    ans = ((H[m] - res1 - res2 + res3) % mod + mod) % mod;
    meow("%lld %lld %lld %lld %lld\n", H[m], res2, res1, res3, ans); 
    printf("%lld\n", ans);
    return 0;
}
数论, 莫比乌斯反演
New Beginning
CF1601B Frog Traveler
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