GCD Convolution

what

the GCD convolution of 2 array a, b consisting of positive integers, is an array d such that:

$$\begin{array}{rl}{d}_g& =\sum _{gcd(a_i,b_j)=g}{a}_i\ast b_j\end{array}$$

resources

• [Tutorial] GCD Convolution - Codeforces
• uses the technique in count the GCD values of all pairs