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Abel's theorem in problems and solutions - School of Mathematics

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Introduction 5<br />

By Viète’s <strong>theorem</strong> <strong>and</strong> are the roots <strong>of</strong> the equation<br />

In this way<br />

where aga<strong>in</strong> <strong>in</strong>dicates one def<strong>in</strong>ed value <strong>of</strong> the square<br />

root. Hence the roots <strong>of</strong> Eq. (6) are expressed by the formula<br />

<strong>in</strong> which for each <strong>of</strong> the three values <strong>of</strong> the first cubic root 4 one must<br />

take the correspond<strong>in</strong>g value <strong>of</strong> the second, <strong>in</strong> such a way that condition<br />

be satisfied.<br />

The obta<strong>in</strong>ed formula is named Cardano’s formula 5 . Substitut<strong>in</strong>g <strong>in</strong><br />

this formula <strong>and</strong> by their expressions <strong>in</strong> terms <strong>of</strong> <strong>and</strong> subtract<strong>in</strong>g<br />

we obta<strong>in</strong> the formula for Eq. (5). After the transformations<br />

we obta<strong>in</strong> the formula for the roots <strong>of</strong> the<br />

generic equation <strong>of</strong> third degree.<br />

Now we exam<strong>in</strong>e the reduced equation <strong>of</strong> fourth degree<br />

(the generic equation is reduced to the previous one by divid<strong>in</strong>g by<br />

By mak<strong>in</strong>g the change <strong>of</strong> variable similarly to the change<br />

made <strong>in</strong> the case <strong>of</strong> the equation <strong>of</strong> third degree, we transform Eq. (9)<br />

<strong>in</strong>to<br />

where <strong>and</strong> are some polynomials <strong>in</strong><br />

We shall solve Eq. (10) by a method called Ferrari’s method 6 . We<br />

transform the left term <strong>of</strong> Eq. (10) <strong>in</strong> the follow<strong>in</strong>g way:<br />

4 See the aforementioned Property 3 <strong>of</strong> complex numbers.<br />

5 G. Cardano (1501-1576) was an Italian mathematician.<br />

6 L. Ferrari (1522–1565) was an Italian mathematician, a pupil <strong>of</strong> Cardano.

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