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May 3, 2011 at 16:44 comment added James Propp Thanks! I see that Norman Alling, in his book "Foundations of Analysis over Surreal Number Fields" (which I obtained after posting my query), proves that the surreal numbers are real-closed by identifying them with formal power series of a suitable kind. He does not (as far as I can tell) characterize ${\bf Cx}$ functorially. Perhaps the right functorial characterization would involve fields equipped with an involution (complex conjugation) that satisfies various properties and in particular induces partial orderings based on real part, imaginary part, and modulus.
May 3, 2011 at 5:59 comment added Qiaochu Yuan The property that adjoining $i$ makes your field algebraically closed is called being real closed and has many, many equivalent formulations: en.wikipedia.org/wiki/Real_closed_field
May 3, 2011 at 5:49 history asked James Propp CC BY-SA 3.0