Firstly, the question was already answered before my comment. This is how I thought about it (perhaps this slow thinking might be helpful for other non-experts, though I am not quite certain about this).

**(1)** In general, first we can observe that, given an ordinal parameter $\alpha$, if an ordinal $x$ is eventually writeable then every ordinal less than $x$ is also eventually writeable.

**(2)** Now observe that for any arbitrary ordinal $\alpha<\omega^L_1$, given the parameter $\alpha$, the code of $\alpha$ is eventually writeable. By code of $\alpha$ I mean the well-order (on $\mathbb{N}$) with order-type $\alpha$.

**(3)** Finally observe that if a real number $r$ (seen as a subset of $\omega$) is eventually writeable using some ordinal parameter $\alpha$ then it is also "ordinal computable" using suitably large ordinal parameters. To be specific, suppose the program with index $e$ when given the parameter $\alpha$ permanently stabilizes its first $\omega$ cells to $r$ beyond some time $T$. Or to put it other way, $T$ is the stabilization time for the real $r$ given the ordinal parameter $\alpha$ and program $\phi_e$.

Now, alternatively, we can think of $r$ as being **computed** with ordinal parameters $\alpha$ and $T$. This re-phrasing can be relevant (and is relevant in the context of this question), due to equivalence of "ordinal computation with parameters" with "constructible subsets of ordinals".

Now the answer to the question is implicit in the above points. That is, suppose that using the parameter $\omega_1$ it was possible to eventually write the code for some ordinal $\geq \omega^L_1$. By point-(1) above, then using the parameter $\omega_1$ it would also be possible to eventually write the code for some ordinal $\omega^L_1$ itself. However, now by point-(3) that would imply that code of $\omega^L_1$ is constructible (which is clearly false).

Hence we conclude that our original assumption was incorrect and hence using the parameter $\omega_1$ it is impossible to eventually write the code for some ordinal $\geq \omega^L_1$. That means that there exists a countable ordinal $\beta<\omega^L_1$ whose code is not eventually writeable using the parameter $\omega_1$. But now, using point-(2), the code for $\beta$ is writeable using the ordinal parameter $\beta$.

antirecognizablein the parameter $\omega_1$ countable in $L$? $\endgroup$2more comments