#P10744. Permutations with Restricted Modular Condition
Permutations with Restricted Modular Condition
Permutations with Restricted Modular Condition
Given an integer n, consider all permutations p = [p1, p2, …, pn] of the numbers 1 to n. We define pn+1 = p1 (i.e. the permutation is considered circular). A permutation is called valid if for every index i (1 ≤ i ≤ n), the condition
\( p_i \bmod p_{i+1} \le 2 \)
holds. Here, the \( \bmod \) operation is the standard remainder operation and all comparisons and computations are done with integers. Output the total number of valid permutations modulo \(10^9+7\).
Note: In a circular sequence, if pi < pi+1, then since p_i mod pi+1 = p_i, we must have p_i \le 2. For indices where p_i \ge p_{i+1}, the condition \(p_i \bmod p_{i+1} \le 2\) must also be verified. It can be shown that for n = 1 the answer is 1, for n = 2 the answer is 2, and for all n \ge 3 the answer equals \(3 \times (n-1)!\) modulo \(10^9+7\).
inputFormat
The input consists of a single line containing an integer n (1 ≤ n).
outputFormat
Output a single integer representing the number of valid permutations modulo \(10^9+7\).
sample
1
1