Abstract :
It is proved that the 3-part of the class number of a quadratic field Q(
√
D) is O(|D|55/112+ )
in general and O(|D|5/12+ ) if |D| has a divisor of size |D|5/6. These bounds follow as results
of nontrivial estimates for the number of solutions to the congruence xa ≡yb modulo q in
the ranges x X and y Y, where a, b are nonzero integers and q is a square-free positive
integer. Furthermore, it is shown that the number of elliptic curves over Q with conductor
N is O(N55/112+ ) in general and O(N5/12+ ) if N has a divisor of size N5/6. These results
are the first improvements to the trivial bound O(|D|1/2+ ) and the resulting bound O(N1/2+ )
for the 3-part and the number of elliptic curves, respectively.
