Upper bounds for the number of number fields with alternating Galois group

Abstract:

Let $N(n, A_n, X)$ be the number of number fields of degree $n$ whose Galois closure has Galois group $A_n$ and whose discriminant is bounded by $X$. By a conjecture of Malle, we expect that $N(n, A_n, X)\sim C_n\cdot X^{\frac {1}{2}} \cdot (\log X)^{b_n}$ for constants $b_n$ and $C_n$. For $6 \leq n \leq 84393$, the best known upper bound is $N(n, A_n, X) \ll X^{\frac {n + 2}{4}}$, by Schmidt’s theorem, which implies there are $\ll X^{\frac {n + 2}{4}}$ number fields of degree $n$. (For $n > 84393$, there are better bounds due to Ellenberg and Venkatesh.) We show, using the important work of Pila on counting integral points on curves, that $N(n, A_n, X) \ll X^{\frac {n^2 - 2}{4(n - 1)}+\epsilon }$, thereby improving the best previous exponent by approximately $\frac {1}{4}$ for $6 \leq n \leq 84393$.