Definition An algebra A is a Family of Sets satisfying the following properties Ω∈A ∀A,B∈A, A−B∈A ∀A,B∈A, A∪B∈A where Ω is a universal set The conditions imply: ∅∈R. Equivalents Ω∈A ∀A∈A, A∁∈A ∀A,B∈F, A∩B∈A Ω∈A ∀A∈A, A∁∈A ∀A,B∈F, A∪B∈A