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#1651. 模运算排列

统计

给定一个自然数 $n$,计算从 $1$ 到 $n$ 的排列 $(p_1, p_2, \dots, p_n)$ 的数量,使得对于每个 $i$ ($1 \le i \le n$),以下性质成立:$p_i \pmod{p_{i+1}} \le 2$,其中 $p_{n+1} = p_1$。

由于该数字可能非常大,请输出其对 $10^9 + 7$ 取模的结果。

输入格式

输入仅一行,包含一个整数 $n$ ($1 \le n \le 10^6$)。

输出格式

输出一个整数,即满足题目条件的排列数量,对 $10^9 + 7$ 取模。

样例

输入 1

1

输出 1

1

输入 2

2

输出 2

2

输入 3

3

输出 3

6

输入 4

4

输出 4

16

输入 5

5

输出 5

40

输入 6

1000000

输出 6

581177467

说明

例如,对于 $n = 4$,你应该统计排列 $[4, 2, 3, 1]$,因为 $4 \pmod 2 = 0 \le 2$,$2 \pmod 3 = 2 \le 2$,$3 \pmod 1 = 0 \le 2$,$1 \pmod 4 = 1 \le 2$。然而,你不应该统计排列 $[3, 4, 1, 2]$,因为 $3 \pmod 4 = 3 > 2$,这违反了题目中的条件。

Editorials

IDTypeStatusTitlePosted ByLast UpdatedActions
#312EditorialOpen题解jiangly2025-12-14 07:02:38View

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