C(21,13,2) = 4

Method of Construction: simple construction: added element to c(v-1,k-1,t) covering

Lower Bound: Schonheim

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