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Eudoxus of Cnidus

Eudoxus (c. 400 B.C.) is the greatest of the ancient mathematicians, surpassed only by Archimedes -- but later. Biographical highlights:

Eudoxus was the most reknown astronomer and mathematician of his day. In astronomy devised an ingenious planetary system based on spheres.

tex2html_wrap_inline176 The spherical earth is at rest at the center.
tex2html_wrap_inline176 Around this center, 27 concentric spheres rotate.
tex2html_wrap_inline176 The exterior one caries the fixed stars,
tex2html_wrap_inline176 The others account for the sun, moon, and five planets.
tex2html_wrap_inline176 Each planet requires four spheres, the sun and moon, three each.

Consider the moon.

Homocentric spheres for the moon


tex2html_wrap_inline176 The description of the motion of the planets is more clever still.

tex2html_wrap_inline176 This model was improved by Callippus by adding more spheres and by Aristotle added to this certain `retrograde" spheres. But all the emendations never accounted for variation of luminosity, which had been observed.

tex2html_wrap_inline176 Eudoxus  also described the constellations and the rising and setting of the fixed stars.

tex2html_wrap_inline176 However, within 50 years the whole theory had to be abandoned.

Eudoxus 's contributions to mathematics include:

There is little question that Eudoxus added to the body of geometric knowledge. Details are scant, but probably his main contributions can be found in Euclid, Books V, VI, and XII.

The Theory of Proportion of Eudoxus  is found as Definition 5 of Euclid, Book V.

Magnitudes are said to be in the same ratio, the first to the second and the third to the fourth, when, if any equimultiples whatever be taken of the first and third, and any equimultiples whatever of the second and fourth, the former equimultiples alike exceed, are alike equal to, or alike fall short of, the latter equimultiples respectively tken in corresponding order.

In modern terms: a/b=c/d if and only if, for all integers m and n , whenever ma<nb then mc<nd, and so on for > and =.

This is tantamount to an infinite process. But it was needed to deal with incommensurables.

The Method of Exhaustion unquestionably helped resolve number of loose ends then extant. It contained as Proposition 1 of Book X.

Two unequal magnitudes being set out, if from the greater there is subtracted a magnitude greater than its half, and from that which is left a magnitude greater than its half, and if this process be repeated continually, there will be left some magnitude which will be less than the lesser magnitude set out.

Let tex2html_wrap_inline212 be givengif. Let tex2html_wrap_inline216 , and tex2html_wrap_inline218 . Let tex2html_wrap_inline220 and tex2html_wrap_inline222 . Continue this process, generating the sequence tex2html_wrap_inline224 , we eventually have tex2html_wrap_inline226 .

How does this differ from our limit concept today?

With this result, Eudoxus  was able to establish following:

Proposition 2. (Book XII) Circles are to one another as the squares on the diameters.
This was proved on the basis of the previous proposition.
Proposition 1. (Book XII) Similar polygons inscribed in circles are to one another as the squares on the diameters.

To prove the Proposition 2, polygonal figures, of indefinitely increasing numbers of sides, are both inscribed and circumscribed in the circle. Assuming Proposition 2 does not hold will lead to the contratiction that the result must be false for the polygons also.


Proof of Proposition 2. Let a and A, d and D be the repectively diameters of the circles. Suppose that


Then there is an a'<a so that


Set tex2html_wrap_inline242 . Let tex2html_wrap_inline244 ( resp. tex2html_wrap_inline246 ) be the inscribed regular polygons of n sides in circle a (resp A). Then


By the method of exhaustion it follows that for large enough n


which implies that


We know that


Thus, since tex2html_wrap_inline264 , it follows that tex2html_wrap_inline266 . But this is impossible, and we have a contradiction.

To complete the proof, it must now be shown that


is also impossible.

This is a double reductio ad absurdum argument, a requirement of this method.

Eudoxus  also demonstrated that the ratios of the volumes of two spheres is as the cubegif of their radii.

On pyramids Eudoxus proved

Proposition 5. (Book XII) Pyramids which are of the same height and have triangular bases are to each other as their bases.

Other propositions are more famousgif:

Proposition. The volume of every pyramid is one third of the prism of on the same base and with the same height.

Proposition. The volume of every cone is one third of the cylinder on the same base and with the same height.

Curiously, the proof is by the method of slabs, familiar to all freshmen.

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Next: About this document

Don Allen
Mon Feb 10 08:03:17 CST 1997