lower bound = 13

C(21,7,2) = 13

Method of Construction: lex covering

Lower Bound: Handbook of Combinatorial Designs section on coverings

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