lower bound = 18

C(25,7,2) = 18

Method of Construction: covering found by a hill-climbing program

Lower Bound: Handbook of Combinatorial Designs section on coverings

  1  2  9 14 16 18 21
  7  9 10 11 13 16 23
  4 12 13 18 20 23 24
  2 10 11 12 14 15 19
  2  6  8 13 14 17 23
  3  5  9 21 22 23 25
  3  5  6  7 15 18 19
  1 15 17 19 20 23 24
  4  8  9 15 16 17 19
  1  2  4  7 20 22 25
  6  9 12 16 22 24 25
 13 14 15 19 21 22 25
  1  3  5  8 12 13 20
  8 10 11 17 18 22 25
  3  4  5  9 14 16 20
  2  3  5 10 11 17 24
  7  8 12 14 17 21 24
  1  4  6 10 11 20 21