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Quadratic Surds

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Why Prove it Again?
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Abstract

This chapter provides an example of how an alternative proof may be used to provide a rational reconstruction of a historical practice. It concerns the following well-known

Theorem: \(\sqrt{n}\) is rational if and only if it is integral, that is, if and only if n is a perfect square.

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Notes

  1. 1.

    As noted already in Zeuthen (1896), pp. 156–157, Euclid’s proof of that proposition is faulty. Proposition VII,20 does, however, follow by reductio from the division algorithm and proposition VII,17 (that \(b/c = ab/ac\) for any natural numbers a, b, c).

References

  • Heath, T.: The Thirteen Books of Euclid’s Elements (3 vols.). Dover, New York (1956)

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  • Itard, J.: Les livres arithmĂ©tique d’Euclide. Hermann, Paris (1961)

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  • Knorr, W.: The Evolution of the Euclidean Elements. A Study of the Theory of Incommensurable Magnitudes and its Significance for Early Greek Geometry. Reidel, Dordrecht (1975)

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  • Zeuthen, H: Geschichte der Mathematik im Altertum und Mittelalter. Andr. Fred. Höst & Sön, Copenhagen (1896)

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Dawson, J.W. (2015). Quadratic Surds. In: Why Prove it Again?. Birkhäuser, Cham. https://doi.org/10.1007/978-3-319-17368-9_4

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