codeforces#P1575G. GCD Festival
GCD Festival
Description
Mr. Chanek has an array $a$ of $n$ integers. The prettiness value of $a$ is denoted as:
$$\sum_{i=1}^{n} {\sum_{j=1}^{n} {\gcd(a_i, a_j) \cdot \gcd(i, j)}}$$
where $\gcd(x, y)$ denotes the greatest common divisor (GCD) of integers $x$ and $y$.
In other words, the prettiness value of an array $a$ is the total sum of $\gcd(a_i, a_j) \cdot \gcd(i, j)$ for all pairs $(i, j)$.
Help Mr. Chanek find the prettiness value of $a$, and output the result modulo $10^9 + 7$!
The first line contains an integer $n$ ($2 \leq n \leq 10^5$).
The second line contains $n$ integers $a_1, a_2, \ldots, a_n$ ($1 \leq a_i \leq 10^5$).
Output an integer denoting the prettiness value of $a$ modulo $10^9 + 7$.
Input
The first line contains an integer $n$ ($2 \leq n \leq 10^5$).
The second line contains $n$ integers $a_1, a_2, \ldots, a_n$ ($1 \leq a_i \leq 10^5$).
Output
Output an integer denoting the prettiness value of $a$ modulo $10^9 + 7$.
Samples
5
3 6 2 1 4
77