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The author studies the equivalence classes under Delta^1_1 isomorphism, otherwise effective Borel isomorphism, between complete separable metric spaces which admit a recursive presentation and he shows the existence of strictly increasing and strictly decreasing sequences as well as of infinite antichains under the natural notion of Delta^1_1-reduction, as opposed to the non-effective case, where only two such classes exist, the one of the Baire space and the one of the naturals.

Polish spaces (Mathematics) --- Metric spaces. --- Isomorphisms (Mathematics)