sol invictus
Philosopher
- Joined
- Oct 21, 2007
- Messages
- 8,613
Since this keeps coming up, here's my attempt at a concise description for how to define sizes of infinity.
Consider two sets (a set is a collection of objects, or numbers, or anything else). We want to determine which set is bigger - meaning which contains more elements. So we do the following - we find a rule that identifies the elements of one set with the elements of the other. We try to find a rule so that each and every element in set 1 identified with a different element of set 2. If we succeed, we know set 1 is smaller than or equal to set 2. If we fail to find any such rule, it means there were more elements in set 1 than in set 2.
For example, X = {1,2}, and Y = {a, b, c}. If my rule is 1->a, 2->b, I've shown |X| <= |Y| (<= is "less than or equal to", |X| means the size of X). If I can prove that there is no rule going from Y->X which satisfies the above (which I could, but won't bother), it shows that |X| < |Y|. That's correct, since in this case |X|=2 and |Y|=3.
Now we try to do exactly the same, only starting from set 2. If we succeed in finding such a rule in both directions, we know the two sets have exactly the same number of elements.
For example, if Z={j,k}, I can take the first rule to be 1->k, 2->j, and the second to be k->2, j->1, which shows |X| = |Z|.
This definition is overkill for finite sets where we can just count how many elements there are. But it's very useful when applied to infinite sets. It immediately tells us, for example, that there are the same number of integers as there are rational numbers (ratios of integers), but there are (infinitely) more real numbers between 0 and 1 than there are integers or rationals.
Consider two sets (a set is a collection of objects, or numbers, or anything else). We want to determine which set is bigger - meaning which contains more elements. So we do the following - we find a rule that identifies the elements of one set with the elements of the other. We try to find a rule so that each and every element in set 1 identified with a different element of set 2. If we succeed, we know set 1 is smaller than or equal to set 2. If we fail to find any such rule, it means there were more elements in set 1 than in set 2.
For example, X = {1,2}, and Y = {a, b, c}. If my rule is 1->a, 2->b, I've shown |X| <= |Y| (<= is "less than or equal to", |X| means the size of X). If I can prove that there is no rule going from Y->X which satisfies the above (which I could, but won't bother), it shows that |X| < |Y|. That's correct, since in this case |X|=2 and |Y|=3.
Now we try to do exactly the same, only starting from set 2. If we succeed in finding such a rule in both directions, we know the two sets have exactly the same number of elements.
For example, if Z={j,k}, I can take the first rule to be 1->k, 2->j, and the second to be k->2, j->1, which shows |X| = |Z|.
This definition is overkill for finite sets where we can just count how many elements there are. But it's very useful when applied to infinite sets. It immediately tells us, for example, that there are the same number of integers as there are rational numbers (ratios of integers), but there are (infinitely) more real numbers between 0 and 1 than there are integers or rationals.
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