@misc{soberon-2011-piercing-balanced-families,
  title = {{Piercing Numbers for Balanced and Unbalanced Families}},
  author = {Luis Montejano and Pablo Soberón},
  year = {2011},
  doi = {10.1007/s00454-010-9295-7},
  note = {Discrete Comput. Geom. 45 (2), 358–364.}
}