Disk covering problem $r(21)$
The smallest value $r(21)$ such that $21$ disks of radius $r(21)$ can be arranged to cover the unit disk.
Lower bound:
Unknown
Upper bound:
${\approx}0.263$
Updates
-
2018
Upper bound: ${\approx}0.263$
Jeremy Tan in 2018
[via Circles Covering Circles, Erich Friedman]