Pell Equation

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# Pell Equation

12 Feb 2012

Diophantine equations are polynomial equations that only admit whole solutions. In this post, I’ll comment on three particular cases and focus on one, known as Pell’s equations.

## Pythagorean triplets

The best known type of Diophantine equations is of the form:

$x ^ 2 + y ^ 2 = z ^ 2$

The integer solutions to this equation are known as Pythagorean triplets, from the Pythagorean theorem regarding the relationship between the lengths of the sides of a right triangle. For the more general case, of the form $x ^ n + y ^ n = z ^ n$ with $n > 2$, the famous Fermat’s last theorem says that there are no whole solutions.

## Bézout’s identity

Another known type is the linear Diophantine equations of two variables, for example, $x$ and $y$, of the form

$ax + by = c$

Where $a, b, c$ are integers. In the case where $c$ is the greatest common factor of $a, b$, we have Bézout’s identity. It is possible to find an integer $x, y$ pair using the extended Euclid’s algorithm.

Note however that there are infinite integer solutions to this equation, since if $x, y$ is a solution, $x ‘= x + b$ and $y’ = y - a$ is also a solution.

## Pell’s equation

I only recently heard about another particular case, known as Pell’s equations, of the form:

$(1) \quad x ^ 2 - ny ^ 2 = 1$

where $n$ is a positive integer. If $n$ does not have an exact root (i.e. $\sqrt(n)$ is not integer), then there are infinite integer solutions $x, y$ (if $n$ has an exact root you can show that the only solution is $x = \ pm 1$ and $y = 0$). We will present an algorithm to find the solutions for this particular case of $n$.

## Continued fractions

A concept used in the algorithm is that of continued fractions, which can be used to approximate an irrational number through fractions.

Given a number $d$, the coefficients of its continued fraction, $a_0, a_1, \ cdots, a_n$ are positive and can be calculated using recurrences. Let $a_0 = \lfloor d \rfloor$ and $r_0 = c$. Then for $i \ge 1$:

$r_i = \frac {1}{r_{i-1} - a_{i-1}}$ $a_i = \lfloor r_i \rfloor$

We can represent a continued fraction by its coefficients, that is, $[a_0, a_1, a_2, \ cdots]$. Incredibly, in the particular case in which the irrational number is of the form $\sqrt{n}$, it can be proved that the coefficients of its continued fraction are periodic, that is $\sqrt{n} = [a_0, \ overline {a_1, a_2, \ cdots, a_{r-1}, a_{r}}]$ and $a_{r} = 2a_0$.

As an example, for $n = 14$ we have $\sqrt{14} = [3, \overline {1,2,1,6}]$

It is possible to explicitly calculate the numerator $p_i$ and the denominator $q_i$ of the continued fraction with $i$ terms, through recurrences:

$\begin{array} {lcl} p_0 & = & a_0 \\ p_1 & = & a_1 a_0 + 1 \\ p_n & = & a_n p_{n-1} + p_{n-2} \end{array}$

and

$\begin{array} {lcl} q_0 & = & 1 \\ q_1 & = & a_1 \\ q_n & = & a_n q_ { n-1} + q_ {n-2} \end{array}$

The fraction $p_i / q_i$ is called the $n$-th convergent. Returning to the example for $n = 14$, we have:

$\begin{array} {llcl} i: & p_i / q_i & & \\ 0: & 3/1 & = & 3.0 \\ 1: & 4 / 1 & = & 4.0 \\ 2: & 11/3 & = & 3.66666666667 \\ 3: & 15/4 & = & 3.75 \\ 4: & 101/27 & = & 3.74074074074 \end{array}$

Where $\sqrt{14}$ is approximately $3.74165769645$

## A solution to the Pell equation

Consider the coefficients of the continued fraction of $\sqrt{n}$ and $r$ the index from which the coefficients are periodic. If $r$ is even, let $x = p_{r-1}$ and $y = q_{r-1}$. Otherwise, be $x = p_{2r-1}$ and $y = q_{2r-1}$. Surprisingly, it’s possible to show that $x, y$ represent the smallest positive integer solution for (1), which we’ll call the fundamental solution.

### Finding the fundamental solution

Given these properties, an algorithm to find a solution for (1), consists of iterating over the $p_i$ and $q_i$ coefficients, until you find $r$ such that $a_r = 2a_0$. According to Lenstra [2], $r \in O (\sqrt{n} \log n)$, that is, it has complexity pseudo-polynomial. Thus, if $d$ is the number of bits needed to represent $n$, we have to $r \in O(2^{d / 2} d)$, that is, even ignoring the complexities of the arithmetic operations in code, our algorithm is exponential in the number of bits.

### Generating more solutions

Given a fundamental solution $(x_1, y_1)$ , Lenstra [2] states that if we sort the solutions by magnitude, then the $k$-th solution $(x_k, y_k)$ is such that

$(2) \quad x_k + \sqrt{n} y_k = (x_1 + \sqrt{n} y_1)^k$

Since both $(x_1, y_1)$ and $(x_k, y_k)$ are solution for (1), we have:

$x_k^2 - ny_k^2 = (x_1^2 - ny_1^2)^k = 1$

Factoring we have,

$(x_k + \sqrt{n} y_k) (x_k - \sqrt{n} y_k) = (x_1 + \sqrt{n} y_1)^k (x_1 - \sqrt{n} y_1)^k$

From (2) we conclude that

$(3) \quad x_k - \sqrt{n} y_k = (x_1 - \sqrt{n} y_1)^k$

Solving for (2) and (3), we arrive at:

$\begin{array} {lcl} x_k & = & \frac {(x_1 + y_1 \sqrt{n})^k + (x_1 - y_1 \sqrt{n})^k} {2} \\ \\ y_k & = & \frac {(x_1 + y_1 \sqrt{n})^k - (x_1 - y_1 \sqrt{n})^k} {2 \sqrt {n}} \end{array}$

### Implementation

Based on the theory presented above, it is simple to write an algorithm. I added a python implementation to my personal number theory library, available on Github.

I found a very interesting post about Pell and Haskell equations here [3]. To practice, I decided to implement my version before looking at the post code. I had to deal with the fact that Haskell does not convert from floating point and integer types automatically. It is necessary to do this explicitly and that is why the fromIntegral() function in the code of this link.

## References

• [1] Wolfram - Pell Equation
• [2] Solving the Pell Equation - HW Lenstra Jr.
• [3] A Foray into Number Theory with Haskell