Constant DEFAULT_DIVIDING_PARAMETER 

pub const DEFAULT_DIVIDING_PARAMETER: usize = 3;

The default number of divisions per objective axis of the Das–Dennis reference lattice — Optuna’s dividing_parameter=3.

Three objectives at p = 3 give 10 reference points; Deb & Jain use p = 12 (91 points) with a population of 92 for that case. The population should be at least the reference-point count for every direction to be able to hold a member.