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These are two examples in which both the subset and the whole set are infinite, and the subset has the same cardinality (the concept that corresponds to size, that is, the number of elements, of a finite set) as the whole; such cases can run counter to one's initial intuition. The set of rational numbers is a proper subset of the set of real ...
Equality between sets can be expressed in terms of subsets. Two sets are equal if and only if they contain each other: that is, A ⊆ B and B ⊆ A is equivalent to A = B. [30] [8] The empty set is a subset of every set: ∅ ⊆ A. [17] If A is a subset of B, but A may not equal to B, then A is called a proper subset of B. This can be written A ...
Subset is denoted by , proper subset by . The symbol ⊂ {\displaystyle \subset } may be used if the meaning is clear from context, or if it is not important whether it is interpreted as subset or as proper subset (for example, A ⊂ B {\displaystyle A\subset B} might be given as the hypothesis of a theorem whose conclusion is obviously true in ...
For instance, had been declared as a subset of , with the sets and not necessarily related to each other in any way, then would likely mean instead of . If it is needed then unless indicated otherwise, it should be assumed that X {\displaystyle X} denotes the universe set , which means that all sets that are used in the formula are subsets of X ...
A set of stamps partitioned into bundles: No stamp is in two bundles, no bundle is empty, and every stamp is in a bundle. The 52 partitions of a set with 5 elements. A colored region indicates a subset of X that forms a member of the enclosing partition. Uncolored dots indicate single-element subsets.
For any set X of real numbers, the interval enclosure or interval span of X is the unique interval that contains X, and does not properly contain any other interval that also contains X. An interval I is a subinterval of interval J if I is a subset of J. An interval I is a proper subinterval of J if I is a proper subset of J.
In his controversial 1878 paper, Cantor explicitly defined the notion of "power" of sets and used it to prove that the set of all natural numbers and the set of all rational numbers are equinumerous (an example where a proper subset of an infinite set is equinumerous to the original set), and that the Cartesian product of even a ...
Since every set is contained in its closure, two separated sets automatically must be disjoint. The closures themselves do not have to be disjoint from each other; for example, the intervals [ 0 , 1 ) {\displaystyle [0,1)} and ( 1 , 2 ] {\displaystyle (1,2]} are separated in the real line R , {\displaystyle \mathbb {R} ,} even though the point ...