A subsingleton generally refers to a subset (of some ambient set AA) having at most one element. That is, it is a subset BB of AA such that any two elements of BB are equal.

Of course, classically any subsingleton is either empty or a singleton, but constructively this need not hold. In a topos, the “object of subsingletons in AA” is the partial map classifier for AA, often denoted A A_\bot.

Revised on May 30, 2010 06:06:16 by Toby Bartels (