# 【题目】Counting Arrays

Educational Codeforces Round 138 (Rated for Div. 2)

D. Counting Arrays

https://codeforces.com/contest/1749/problem/D

2 seconds / 512 megabytes

standard input / standard output

### Problem

Consider an array $a$ of length $n$ with elements numbered from $1$ to $n$. It is possible to remove the $i$-th element of $a$ if $gcd(a_i, i) = 1$, where gcd denotes the greatest common divisor. After an element is removed, the elements to the right are shifted to the left by one position.

An array $b$ with $n$ integers such that $1 \le b_i \le n - i + 1$ is a removal sequence for the array $a$ if it is possible to remove all elements of $a$, if you remove the $b_1$-th element, then the $b_2$-th, ..., then the $b_n$-th element. For example, let $a = [42, 314]$:

• $[1, 1]$ is a removal sequence: when you remove the $1$-st element of the array, the condition $gcd(42, 1) = 1$ holds, and the array becomes $[314]$; when you remove the $1$-st element again, the condition $gcd(314, 1) = 1$ holds, and the array becomes empty.
• $[2, 1]$ is not a removal sequence: when you try to remove the $2$-nd element, the condition $gcd(314, 2) = 1$ is false.

An array is ambiguous if it has at least two removal sequences. For example, the array $[1, 2, 5]$ is ambiguous: it has removal sequences $[3, 1, 1]$ and $[1, 2, 1]$. The array $[42, 314]$ is not ambiguous: the only removal sequence it has is $[1, 1]$.

You are given two integers n and m. You have to calculate the number of ambiguous arrays $a$ such that the length of $a$ is from $1$ to $n$ and each $a_i$ is an integer from $1$ to $m$.

### Input

The only line of the input contains two integers $n$ and $m$ ($2 \le n \le 3 \cdot 10^5$; $1 \le m \le 10^{12}$).

### Output

Print one integer — the number of ambiguous arrays $a$ such that the length of $a$ is from $1$ to $n$ and each $a_i$ is an integer from $1$ to $m$. Since the answer can be very large, print it modulo $998244353$.

### Examples

InputOutput
2 36
4 226
4 61494
1337 424242424242119112628

### 题解

$\vdots$

$$\prod_{i=1}^{l}{\lfloor\frac{m}{pm_i}\rfloor}$$

$$\sum_{l=1}^{n}{\prod_{i=1}^{l}{\lfloor\frac{m}{pm_i}\rfloor}}$$

### 代码

• 代码中使用线性筛法生成质数表，时间复杂度 $O(n)$.
• 请注意代码中每一个取模的地点，漏掉的话，很有可能出现溢出问题（尤其是 cout 输出时取模）
#include <bits/stdc++.h>

using namespace std;

typedef long long ll;
const ll MOD = 998244353, MAXN = 3e5 + 10;
bool not_prime[MAXN];
int prime[MAXN], idx;
void init_prime()
{
not_prime[0] = not_prime[1] = true;
for (int i = 2; i < MAXN; i++)
{
if (!not_prime[i])
prime[idx++] = i;
for (int j = 0; j < idx && prime[j] * i < MAXN; j++)
{
not_prime[prime[j] * i] = true;
if (!(i % prime[j]))
break;
}
}
}

int main()
{
init_prime();
ll n, m;
cin >> n >> m;
ll ans = 0, cnt = 1, prime_mul = 1;
for (ll i = 1; i <= n; i++)
{
if (!not_prime[i])
prime_mul *= i;
if (prime_mul > m)
break;
cnt = cnt * (m / prime_mul % MOD) % MOD;
ans = (ans + cnt) % MOD;
}
ll all = 0, pow_m = 1;
for (ll i = 1; i <= n; i++)
{
pow_m = pow_m * (m % MOD) % MOD;
all = (all + pow_m) % MOD;
}
cout << (all - ans + MOD) % MOD << endl;
return 0;
}