Geodesic
Perfect subsets of invariant CA-sets
Matematičeskie zametki, Tome 77 (2005) no. 3, pp. 334-338

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The familiar theorem that any $\Sigma^1_2(a)$-set $X$ of real numbers (where $a$ is a fixed real parameter) not containing a perfect kernel necessarily satisfies the condition $X\subseteq\mathbf L[a]$ is extended to a wider class of sets, with countable ordinals allowed as additional parameters in $\Sigma^1_2(a)$-definitions. 
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     author = {V. G. Kanovei and V. A. Lyubetskii},
     title = {Perfect subsets of invariant {CA-sets}},
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     number = {3},
     year = {2005},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/MZM_2005_77_3_a1/}
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V. G. Kanovei; V. A. Lyubetskii. Perfect subsets of invariant CA-sets. Matematičeskie zametki, Tome 77 (2005) no. 3, pp. 334-338. http://geodesic.mathdoc.fr/item/MZM_2005_77_3_a1/