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If one were to verify that

$$ \sqrt{2} < 3 $$

would the underlying formalisation require a logic more expressive than first-order? Or, is FOL sufficient since real numbers can be formalised in set theory?

How about evaluating differentials, like

$$ \frac{d}{dy} (3x+2) $$

what how expressive does the logic need to be?


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1 Answer 1

Comparing particular algebraic numbers (localized away from their Galois conjugates by inequalities, such as the positive root of $X^2=2$) can be done in any theory that supports arithmetic calculation. For example, the first-order theory of real-closed fields is decidable and that includes decisions about statements of the form $a > b$ comparing two real algebraic numbers.

Model theory of differential (and difference) fields and rings usually takes place in a first-order logic context similar to fields. Theorems are proved by second-order methods but the object of study is a first-order theory.

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I see. Why are theorems proved by second-order methods? Can you name some example objects of study that are second or higher order theories? –  Kar Sep 7 '11 at 0:31

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