hiː kəntɹˈɪbjuːɾᵻd ɡɹˈeɪtli tə ðə θˈiəɹi ʌv ˌænɐlˈɪɾɪk sˈɛts, sˈʌmtaɪmz kˈɔːld ˈæftɚ hˌɪm, ɐ kˈaɪnd əvə sˈɛt ʌv ɹˈiːəlz ðæt ɪz dᵻfˈaɪnəbəl vˈaɪə tɹˈiːz.
He contributed greatly to the theory of analytic sets, sometimes called after him, a kind of a set of reals that is definable via trees.