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Hyperelliptic Curves, Continued Fractions and Somos Sequences

Hyperelliptic Curves, Continued Fractions and Somos Sequences

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The surprising integral<br />

Z X 6t dt<br />

p<br />

t4 + 4t3 − 6t2 + 4t + 1<br />

“<br />

= log X 6 + 12X 5 + 45X 4 + 44X 3 − 33X 2 + 43<br />

+ (X 4 + 10X 3 + 30X 2 p<br />

+ 22X − 11) X 4 + 4X 3 − 6X 2 ”<br />

+ 4X + 1<br />

is a nice example of a class of pseudo-elliptic integrals<br />

Z X f (t)dt<br />

p = log<br />

D(t) ` q<br />

a(X) + b(X) D(X) ´ . (∗)<br />

Here we take D to be a monic polynomial defined over Q, of even<br />

degree 2g + 2, <strong>and</strong> not the square of a polynomial; f , a, <strong>and</strong> b denote<br />

appropriate polynomials. We suppose a to be nonzero, say of degree<br />

m at least g + 1. We will see that necessarily deg f = g , that<br />

deg b = m − g − 1, <strong>and</strong> that f has leading coefficient m.<br />

In the example, m = 6 <strong>and</strong> g = 1.<br />

5

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