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2010 | OriginalPaper | Chapter
Cardinal Number and Ordinal Number
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Recall from Chapter 13 that we write card
A
= card
B
if there is a bijection
A
→
B
. Then we say that
A
and
B
have the same cardinal number. We write card
A
≤ card
B
if there is an injection
A
→
B
. In this latter case we say that the cardinal number of
A
is less than or equal to the cardinal number of
B
. Recall that card
N
≤ card
R
but card
N
≠ card
R
.