Given integers $N$ and $D$, find how many pairs of integers $(i, j)$ such that $1 \le i \le j \le N$ have the greatest common divisor exactly $D$.
I know it involves Mobius inversion somehow, but I can't seem to get the exact expression. The constraints on $N$ and $D$ are quite large (of the order of $10^8$), so it should be a direct formula.
Since $D$ must divide both $i$ and $j$, you can divide everything through by $D$ immediately and reduce to: how many pairs $(k,\ell)$ of integers such that $1\le k\le \ell \le M$ have $\gcd(k,\ell)=1$, where $M=\lfloor N/D \rfloor$.
By the characteristic property of the Möbius function, the answer is $$ \sum_{1\le k\le\ell\le M} \sum_{d\mid\gcd(k,\ell)} \mu(d) = \sum_{1\le d\le M} \mu(d) \sum_{\substack{k\le\ell\le M \\ d\mid k,\, d\mid\ell}} 1 = \sum_{d\le M} \mu(d) \frac12 \bigg\lfloor \frac Md \bigg\rfloor \bigg( \bigg\lfloor \frac Md \bigg\rfloor + 1 \bigg). $$ This is approximately $$ \frac{M^2}2 \sum_{d=1}^\infty \frac{\mu(d)}{d^2} = \frac{3M^2}{\pi^2}. $$
