Cardinalities, aleph, and beth
"K Heuer" <47ynsva02-O/[email protected]> 23 Aug 2003 02:40:34 -0000
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In <http://www.ii.com/math/ch/>, Appendix 2, you quote Gamow as saying "there are aleph_2 different curves." You correctly point out that he's saying aleph_2 when he really means 2^2^aleph_0, but you missed a different mistake: the number of curves is actually only 2^aleph_0, i.e., equal to the number of points, not greater. A correct statement would be "there are 2^2^aleph_0 different functions from R to R." (Most of those functions are discontinuous, and hence their graphs are not curves.)
Incidentally, this first confusion -- using aleph_1 to mean the continuum and aleph_2 to mean its power set -- would be reduced, I believe, if the beth notation were more widely known and used. I was never formally taught this notation, but my understanding is that beth_0 = aleph_0, beth_(k+1) is 2^beth_k, and for a limit ordinal l, beth_l = sup { beth_k | k < l }. Then CH can be stated "beth_1 = aleph_1", and GCH can be stated "beth_k = aleph_k for all ordinals k."
Karl Heuer