T. Ono raised some problems on relations between Ono's numbers $p_D$ and the class numbers $h_D$ of imaginary quadratic fields. In this paper we give an upper bound for $p_D$. The upper bound contributes to one of the problems.
Fumio Sairaiji. Kenichi Shimizu. "A note on Ono's numbers associated to imaginary quadratic fields." Proc. Japan Acad. Ser. A Math. Sci. 77 (2) 29 - 31, Feb. 2001. https://doi.org/10.3792/pjaa.77.29