  We show that there is a positive constant $c_0$ such that \[\sum_{n\le
x}\mu^2(n^2+1)c_0x+O_{\varepsilon}(x^{7/12+\varepsilon})\] for any fixed
$\varepsilon>0$. This improves a result of Estermann [3] from 1931, in which
the error term had an exponent 2/3. The proof involves counting rational points
near an algebraic curve, which is done via the "determinant method."

Number Theory