洛谷 6156 简单题

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yysy，确实是个简单题，而且很套路（

首先显然 \(f = \mu^2\)。
那么 \[ \begin{aligned} \sum\limits_{i=1}^n \sum\limits_{j=1}^n (i+j)^k \mu^2(\gcd(i,j))\gcd(i,j) &= \sum\limits_{d=1}^n \mu^2(d) d \sum\limits_{i=1}^n \sum\limits_{j=1}^n [\gcd(i,j)=d] (i+j)^k \\ &= \sum\limits_{d=1}^n \mu^2(d) d^{k+1} \sum\limits_{i=1}^{\left\lfloor\frac nd\right\rfloor} \sum\limits_{j=1}^{\left\lfloor\frac nd\right\rfloor} [\gcd(i,j)=1] (i+j)^k \\ &= \sum\limits_{d=1}^n \mu^2(d) d^{k+1} \sum\limits_{i=1}^{\left\lfloor\frac nd\right\rfloor} \sum\limits_{j=1}^{\left\lfloor\frac nd\right\rfloor} (i+j)^k \sum\limits_{u|\gcd(i,j)} \mu(u) \\ &= \sum\limits_{d=1}^n \mu^2(d) d^{k+1} \sum\limits_{u=1}^{\left\lfloor\frac nd\right\rfloor} \mu(u)u^k \sum\limits_{i=1}^{\left\lfloor\frac n{du}\right\rfloor} \sum\limits_{j=1}^{\left\lfloor\frac n{du}\right\rfloor} (i+j)^k \end{aligned} \]

注意到可以数论分块套数论分块，则这一部分的复杂度为 \(O(n^{3/4})\)。
则我们需要求得 \(\mu^2(i)i^{k+1},\mu(i)i^k\) 的前缀和，并对于所有 \(n'\) 求出 \(\sum\limits_{i=1}^{n'} \sum\limits_{j=1}^{n'} (i+j)^k\)。
前两个线性筛出所有的 \(\mu(i),i^k\) 即可计算，最后一个考虑观察其关于 \(n'\) 差分后的结果： \[ \begin{aligned} \sum\limits_{i=1}^{n+1} \sum\limits_{j=1}^{n+1} (i+j)^k - \sum\limits_{i=1}^n \sum\limits_{j=1}^n (i+j)^k &= \sum\limits_{i=1}^n \sum\limits_{j=1}^{n+1} (i+j)^k + \sum\limits_{j=1}^{n+1} (n+1+j)^k - \sum\limits_{i=1}^n \sum\limits_{j=1}^n (i+j)^k \\ &= \sum\limits_{i=1}^n \left(\sum\limits_{j=1}^{n+1} (i+j)^k - \sum\limits_{j=1}^n (i+j)^k\right) + \sum\limits_{j=1}^{n+1} (n+1+j)^k \\ &= \sum\limits_{i=1}^n (i+n+1)^k + \sum\limits_{j=1}^{n+1} (n+1+j)^k \end{aligned} \]

于是这个也可以 \(O(n)\) 递推求得。
总时间复杂度 \(O(n)\)。

代码： 

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#include <cstdio>
#define add(x,y) (x + y >= mod ? x + y - mod : x + y)
#define dec(x,y) (x < y ? x - y + mod : x - y)
using namespace std;
const int N = 1e7;
const int mod = 998244353;
inline int fpow(int a,int b)
{
    int ret = 1;
    for(;b;b >>= 1)
        (b & 1) && (ret = (long long)ret * a % mod),a = (long long)a * a % mod;
    return ret;
}
int n,k;
int vis[N + 5],cnt,prime[N + 5],mu[N + 5],pw[N + 5];
int sum[3][N + 5],ans;
int main()
{
    long long x;
    scanf("%d%lld",&n,&x),k = x % (mod - 1),n <<= 1,mu[1] = pw[1] = 1;
    for(register int i = 2;i <= n;++i)
    {
        if(!vis[i])
            mu[prime[++cnt] = i] = mod - 1,pw[i] = fpow(i,k);
        for(register int j = 1;j <= cnt && i * prime[j] <= n;++j)
        {
            vis[i * prime[j]] = 1,pw[i * prime[j]] = (long long)pw[i] * pw[prime[j]] % mod;
            if(!(i % prime[j]))
                break;
            mu[i * prime[j]] = dec(0,mu[i]);
        }
    }
    n >>= 1;
    for(register int i = 1;i <= n;++i)
        sum[0][i] = (sum[0][i - 1] + (long long)mu[i] * mu[i] % mod * pw[i] % mod * i % mod) % mod,
        sum[1][i] = (sum[1][i - 1] + (long long)mu[i] * pw[i] % mod) % mod;
    n <<= 1;
    for(register int i = 1;i <= n;++i)
        pw[i] = (pw[i - 1] + pw[i]) % mod;
    n >>= 1;
    for(register int i = 1;i <= n;++i)
        sum[2][i] = add(sum[2][i - 1],dec(pw[i << 1],pw[i])),
        sum[2][i] = add(sum[2][i],dec(pw[(i << 1) - 1],pw[i]));
    for(register int L = 1,R,s;L <= n;L = R + 1)
    {
        R = n / (n / L),s = 0;
        for(register int l = 1,r,m = n / L;l <= m;l = r + 1)
        {
            r = m / (m / l);
            s = (s + (long long)(sum[1][r] - sum[1][l - 1] + mod) * sum[2][m / l] % mod) % mod;
        }
        ans = (ans + (long long)(sum[0][R] - sum[0][L - 1] + mod) * s % mod) % mod;
    }
    printf("%d\n",ans);
}