"Improving Dense Packings of Equal Disks in a Square"
by David W. Boll, Jerry Donovan , Ronald L. Graham and Boris D. Lubachevsky
The portion of the bibliography of this paper which is accessible electronically is:
[GL] R. L. Graham and B. D. Lubachevsky,
Repeated Patterns of Dense Packings of Equal Disks in a Square
Electronic Journal of Combinatorics