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The cardinality of 15 is equal to the number of elements in the set. Since 15 is only one number, its cardinality is 1.
Cardinality is simply the number of elements of a given set. You can use the cardinality of a set to determine which elements will go into the subset. Every element in the subset must come from the cardinality of the original set. For example, a set may contain {a,b,c,d} which makes the cardinality 4. You can choose any of those elements to form a subset. Examples of subsets may be {a,c} {a, b, c} etc.
Two sets are equivalent if they have the same cardinality. For finite sets this means that they must have the same number of distinct elements. For infinite sets, equal cardinality means that there must be a one-to-one mapping from one set to the other. This can lead to some counter-intuitive results. For example, the cardinality of the set of integers is the same as the cardinality of the set of even integers although the second set is a proper subset of the first. The relevant mapping is x -> 2x.
An empty set is not a proper subset of an empty set.An empty set is not a proper subset of an empty set.An empty set is not a proper subset of an empty set.An empty set is not a proper subset of an empty set.
No. The number of subsets of that set is strictly greater than the cardinality of that set, by Cantor's theorem. Moreover, it's consistent with ZFC that there are two sets which have different cardinality, yet have the same number of subsets.