Home > RHUMJ > Vol. 7 (2006) > Iss. 2

#### Abstract

In 1659, John Pell and Johann Rahn wrote a text which explained how to find all integer solutions to the quadratic equation u2 - d v2 = 1. In 1909, Axel Thue showed that the cubic equation u3 - d v3 = 1 has finitely many integer solutions, so it remains to examine their rational solutions. We explain how to find "large" rational solutions i.e., a sequence of rational points (un, vn) which increase without bound as n increases without bound. Such cubic equations are birationally equivalent to elliptic curves of the form y2 = x3 - D. The rational points on an elliptic curve form an abelian group, so a "large" rational point (u,v) maps to a rational point (x,y) of "approximate" order 3. Following an idea of Zagier, we explain how to compute such rational points using continued fractions of elliptic logarithms. We divide our discussion into two parts. The first concerns Pell's quadratic equation. We give an informal discussion of the history of the equation, illuminate the relation with the theory of groups, and review known results on properties of integer solutions through the use of continued fractions. The second concerns the more general equation uN - d vN = 1. We explain why N = 3 is the most interesting exponent, present the relation with elliptic curves, and investigate properties of rational solutions through the use of elliptic integrals. This project was completed at Miami University, in Oxford, OH as part of the Summer Undergraduate Mathematical Sciences Institute (SUMSRI).

#### Sponsor

Edray Goins, Department of Mathematics, Purdue University egoins@math.purdue.edu

#### Recommended Citation

Cunningham, Jarrod Anthony; Ho, Nancy; Lostritto, Karen; Middleton, Jon Anthony; and Thomas, Nikia Tenille
(2006)
"On Large Rational Solutions of Cubic Thue Equations: What Thue Did to Pell,"
*Rose-Hulman Undergraduate Mathematics Journal*: Vol. 7
:
Iss.
2
, Article 6.

Available at:
http://scholar.rose-hulman.edu/rhumj/vol7/iss2/6