It seems that one can construct ordinals from bottom up by successively introducing a new symbol each time a limit is taken: $$1,\ 2,\ \ldots,\ \omega,\ \omega +1,\ \omega +2,\ \ldots,\ \omega\cdot 2,\ \omega\cdot 2 +1,\ \ldots,\ \omega^{2},\ \ldots,\ \omega^{3},\ \ldots\ \omega^{\omega},\ \ldots,\ \omega^{\omega^{\omega}},\ \ldots, \epsilon_{0},\ \ldots$$ Can this be taken as a (mechanical) definition of ordinals? More abstract definitions like "an ordinal is a transitive well-ordered set satisfying certain properties" are much more appealing to me. Is this mechanical definition sufficient to prove things like "each well-ordered set is order isomorphic to exactly one ordinal?"
Mechanical definition of ordinals
2
$\begingroup$
elementary-set-theory
ordinals
-
4What do you do when you reach an uncountable ordinal - do you have an uncountable number of symbols to choose from? What if we just use each ordinal as a symbol for itself? – 2011-10-11
-
3Worse yet, the first uncountable ordinal $\omega_1$ _cannot_ be reached as the limit of a countable sequence of smaller ordinals. So your process will give you _at most_ the countable ordninals. – 2011-10-11
-
0@Henning: this is an argument in favor of taking each ordinal as a symbol for itself. – 2011-10-11
-
1@Carl, they are awfully hard to write down on paper (except by use of other symbols, and even then we don't get most of them), which strikes me as a rather basic requirement for symbols. – 2011-10-11
-
0It is perhaps off-topic here, but maybe this is why induction to [$\varepsilon_0$](http://en.wikipedia.org/wiki/%CE%95%E2%82%80) is (by some people) considered "almost finitistic". (I've seen this mentioned in connection with [Hilbert's program](http://en.wikipedia.org/wiki/Hilbert%27s_program).) Ordinals up to $\varepsilon_0$ are basically ordinals we can "imagine to write down". – 2011-10-11
-
2In fact, the intent of the OP's method won't even get you to a nonrecursive ordinal, but it will get you to things that make $\varepsilon_{0}$ pale into insignificance (e.g. $\Gamma_{0}$, $\Gamma_{\varepsilon_{0}}$, the Bachmann-Howard ordinal, etc.). See http://en.wikipedia.org/wiki/Recursive_ordinal and http://en.wikipedia.org/wiki/Large_countable_ordinal – 2011-10-11