Cardinal Number and Ordinal Number
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.
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