On sparse countably infinite Steiner triple systems

We give a general construction for Steiner triple systems on a countably infinite point set and show that it yields 2   ℵ   0nonisomorphic systems all of which are uniform and r ‐sparse for all finite r ⩾4. © 2009 Wiley Periodicals, Inc. J Combin Designs 18: 115–122, 2010