【题解】Luogu-P3768 简单的数学题

P3768 简单的数学题

Description

• 给定整数 $n,p$，请求出

$$\left(\sum_{i = 1}^{n} \sum_{j = 1}^n ij \gcd(i, j) \right) \bmod p$$

• 对于 $100\%$ 的数据，$n\le 10^{10}, 5 \times 10^8 \le p \le 1.1 \times 10^9$ 且 $p\in \mathbb{P}$。

Solution

$$\begin{aligned} \sum_{i = 1}^n \sum_{j = 1}^n ij \gcd(i, j) & = \sum_{i = 1}^n \sum_{j = 1}^n ij \sum_{d\mid \gcd(i, j)} \varphi(d) \\ & = \sum_{d = 1}^n \varphi(d) \sum_{i = 1}^n i [d\mid i] \sum_{j = 1}^n j [d\mid j] \\ & = \sum_{d = 1}^n \varphi(d) \sum_{i = 1}^{\left\lfloor\frac{n}{d}\right\rfloor} i d \sum_{j = 1}^{\left\lfloor\frac{n}{d}\right\rfloor} j d \\ & = \sum_{d = 1}^n \varphi(d) d^2 S\left(\left\lfloor\dfrac{n}{d}\right\rfloor\right)^2 \end{aligned}$$

杜教筛 $\varphi(d) \cdot d^2$ 即可。

杜教筛时间复杂度为 $\Omicron(n^{\frac{2}{3}})$，整除分块时间复杂度为 $\Omicron(\sqrt{n})$，总时间复杂度为 $\Omicron(n^{\frac{2}{3}})$。

Code

// 18 = 9 + 9 = 18.
#include <iostream>
#include <cstdio>
#include <map>
#define Debug(x) cout << #x << "=" << x << endl
#define int __int128
using namespace std;

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

void write(int x)
{
	if (x > 9)
	{
		write(x / 10);
	}
	putchar(x % 10 ^ 48);
}

int p, inv2, inv6, inv4;

const int MAXN = 4641588 + 5;
const int N = 4641588;

int pr[MAXN], phi[MAXN], sum[MAXN];
bool vis[MAXN];

void pre()
{
	phi[1] = sum[1] = 1;
	for (int i = 2; i <= N; i++)
	{
		if (!vis[i])
		{
			pr[++pr[0]] = i;
			phi[i] = (i - 1) % p;
		}
		for (int j = 1; j <= pr[0] && i * pr[j] <= N; j++)
		{
			vis[i * pr[j]] = true;
			if (i % pr[j] == 0)
			{
				phi[i * pr[j]] = phi[i] * pr[j] % p;
				break;
			}
			phi[i * pr[j]] = phi[i] * phi[pr[j]] % p;
		}
		sum[i] = (sum[i - 1] + phi[i] * i % p * i % p) % p;
	}
}

int qpow(int a, int b)
{
	int base = a % p, ans = 1;
	while (b)
	{
		if (b & 1)
		{
			ans = ans * base % p;
		}
		base = base * base % p;
		b >>= 1;
	}
	return ans;
}

int inv(int a)
{
	return qpow(a, p - 2);
}

int S1(int n)
{
	n %= p;
	return n * (n + 1) * inv2 % p;
}

int S2(int n)
{
	n %= p;
	return n * (n + 1) % p * (2 * n % p + 1) % p * inv6 % p;
}

int getS2(int l, int r)
{
	return ((S2(r) - S2(l - 1)) % p + p) % p;
}

int S3(int n)
{
	n %= p;
	return n * n % p * (n + 1) % p * (n + 1) % p * inv4 % p;
}

map<int, int> dp;

int sublinear(int n)
{
	if (n <= N)
	{
		return sum[n];
	}
	if (dp.find(n) != dp.end())
	{
		return dp[n];
	}
	int res = S3(n);
	for (int l = 2, r; l <= n; l = r + 1)
	{
		int k = n / l;
		r = n / k;
		res = ((res - getS2(l, r) * sublinear(k) % p) % p + p) % p;
	}
	return dp[n] = res;
}

int getsum(int l, int r)
{
	return ((sublinear(r) - sublinear(l - 1)) % p + p) % p;
}

int block(int n)
{
	int res = 0;
	for (int l = 1, r; l <= n; l = r + 1)
	{
		int k = n / l;
		r = n / k;
		res = (res + getsum(l, r) * S1(k) % p * S1(k) % p) % p;
	}
	return res;
}

signed main()
{
	int n;
	p = read(), n = read();
	inv2 = inv(2), inv6 = inv(6), inv4 = inv(4);
	pre();
	write(block(n));
	putchar('\n');
	return 0;
}