Nicholas J. Cavenagh Diane Donovan Emine Sule Yazici Minimal homogeneous Steiner 2-(v, 3) trades. 741-752 2008 308 Discrete Mathematics 5-6 http://dx.doi.org/10.1016/j.disc.2007.07.060 db/journals/dm/dm308.html#CavenaghDY08