In mathematics, a multiset (aka bag or mset) is a generalization of the concept of a set that, unlike a set, allows multiple instances of the multiset's elements. For example, {a, a, b} and {a, b} are different multisets although they are the same set. However, order does not matter, so {a, a, b} and {a, b, a} are the same multiset.
https://en.wikipedia.org/wiki/Multiset
Let {1,2,3,4} be a set of four natural numbers.
Let multiset A = {{1,2,3,4}, {3,1,2,4}}
In terms of multiset A {1,2,3,4} and {3,1,4,2} are not the same member even if they have the same members (where order does not matter), which means that they are independent of each other under A.
So under A we can make a shift of the mapping between {1,2,3,4} and {3,1,4,2} (for example) as follows:
But it does not matter in case of finite number of members, since the number of the shifted members is equal in both sides, so the mapping above is actually the following mapping:
Is this fact holds also among infinite sets, which are independent members of a given multiset?
Let {1,2,3,...} be the set of all natural numbers (order does not matter).
Let multiset S = {{1,2,3,4,...}, {8,2,3,5,...}} (S has two independent copies of the set of all natural numbers, where order does not matter).
So under S we can make a shift of the mapping between {1,2,3,4,...} and {8,2,3,5,...} (for example) as follows:
Code:
1,2,3,4,5,6,...
↓ ↓ ↓ ↓
8,2,3,5,...
As we have seen above, this kind of shift does not matter in case of finite number of members, but in this case the shift is done among members of two independent infinite sets under multiset S, and one discovers that there can be also no bijection between two infinite sets under a given multiset even if they have the same members.
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In this example |{8,2,3,5,...}| < |{1,2,3,4,...}| by 2, which means that the notion that finite cardinality has no impact on infinite cardinality, does not necessarily hold in case of multisets, or more generally, the notion that the set of all natural numbers must have a fixed cardinality (notated as ℵ
0) does not necessarily hold in this context.