Math Notes Singleton Sets


A singleton set is a set that contains only one member. Some theorems that apply to singleton sets are not generally true for all sets, so the knowing that a set is a singleton set can be important in proving theorems.


Suppose that the element a is a member of the universal set U. The set {a} is a singleton set. A singleton set represents a set such that all members of the set equal the same element. Thus, a singleton set may be defined as the following axiom.


Axiom 1: {a} = {x|x=a:x}.


A singleton set is identified by the member associated with the set. If it can be determined that an element is a member of a singleton set, then the element must be the same as the element associated with the singleton set. A simple result of this theorem is that an element is a member of the singleton set associated with the set.


Theorem 1: b {a} b = a


Corollary: x {x}


For each property of an element, there should be a corresponding property of the singleton set associated with the element.


Theorem 2: a P {a} P


Some properties of singleton sets are not generally true for all sets. Theorem 3 is an equivalence expression for singleton sets, but only an implication if generalized to all sets.


Theorem 3: {a} P {a} Q {a} (P Q)


A singleton set is a member of a set or the sets complement. This property is not generally true of sets with two or more elements.


Theorem 4: {x} B {x} ~B





Construct a Venn diagram that demonstrates why the expression in Theorem 3 is only an implication when generalized to include all sets.




P = {1,2,3}

Q = {4,5,6}


Construct a set R that is not a subset of either set P or set Q, but is a subset of the union of P and Q.





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Ganong, Rick 1999, Course notes and comments from M1090 and M2090 math and logic courses,


Gries, David and Schneider, Fred B., 1993, A Logical Approach to Discrete Math, New York, NY: Springer-Verlag


Hu Sze-Tsen, 1963, Elements of Modern Algebra, San Francisco: Holden-Day, Inc.