EN
We study the sum τ of divisors of the quadratic form m₁² + m₂² + m₃². Let
$S₃(X) = ∑_{1≤m₁,m₂,m₃≤X} τ(m₁²+m₂²+m₃²)$.
We obtain the asymptotic formula
S₃(X) = C₁X³logX + C₂X³ + O(X²log⁷X),
where C₁,C₂ are two constants. This improves upon the error term $O_ε(X^{8/3+ε})$ obtained by Guo and Zhai (2012).