C(21,9,2) = 7

Method of Construction: simple construction: multiple of smaller covering

Lower Bound: Schonheim

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