A power-saving asymptotic for a Möbius-weighted GCD quadratic form

Abstract

For the Möbius-weighted quadratic form $T(N)=\sum_{m,n\leq N}μ(m)μ(n)(m,n)^2/(mn)$, we prove that $T(N)=cN+O_{\varepsilon}(N^{2/3+\varepsilon})$ for every $\varepsilon>0$, where $c>0$ is given by an absolutely convergent series of finite Möbius sums multiplied by a convergent Euler product. The proof combines the classical Jordan-totient decomposition of the GCD kernel with a mean-value estimate that is uniform in a coprimality parameter. Grouping the resulting squares according to $\lfloor N/d\rfloor$ gives the error term and a nonnegative series for $c$.