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The disk covering problem asks for the smallest real number
The best solutions known to date are as follows:
Method
The following picture shows an example of a dashed disk of radius 1 covered by six solid-line disks of radius ~0.6. One of the covering disks is placed central and the remaining five in a symmetrical way around it.
Similar arrangements of six, seven, eight respectively nine disks around a central disk all having same radius result in the best known layout strategies for r(7), r(8), r(9), respectively r(10). The corresponding angles θ are written in the "Symmetry" column in the above table. Pictures showing these arrangements can be found at Friedman, Erich. "circles covering circles". Retrieved 2016-05-04.