Minimal Critical Set of a Room Square of order 7
Chaudhry G R and Seberry J, Minimal Critical Set of a Room Square of order 7, Bull. Inst. of Combin. and its Applications 20(1997), 90.
A Room square R of order r is an r x r array each of whose cells may either be empty or contain an unordered pair of objects 0,1,2,...,r, subject to the following conditions:
(i) each of the objects 0,1,2.....r occurs precisely once in each row of R and precisely once in each column of R, and
(ii) every possible unordered pair of objects occurs precisely once in the whole array.
Ghulam R. Chaudhry and Jennifer Seberry. "Minimal Critical Set of a Room Square of order 7" Faculty of Informatics - Papers (1997).
Available at: http://works.bepress.com/jseberry/332