Hi mathsyperson and Jane;

Continued fractions are used to calculate the smallest solution.
For instance to solve:

You start by computing the continued fraction of √14

The sequence is periodic with length 4 (1,2,1,6...) The nice part is that a theorem by
Lagrange assures us that every square root like this will always have a repeating
form.

Compute the convergents of the √14. These are done by 2 recurence formulae or matrix multiplication

You pick the 4th one in the sequence 15/4 and that is the smallest non trivial answer.

This example is simple enough to get using this theorem. For any positive integer d, if d+2 is a perfect square,
then d+1 is the first solution to Pell's Equation for x. I haven't seen a proof to this, just some web page.

For harold the saxon problem:

Compute the continued fraction of √61

The sequence is periodic with length 11 (1,4,3,1,2,2,1,3,4,1,14...)

Compute the convergents of the continued fraction:

So we pick the 11 term which is 29718/3805 but

which is incorrect. So we try the next 11th (22nd term) term which is

So this is the smallest solution that is not trivial.

Here is a page to solve these and many tougher types of diophantine equtions.
http://www.alpertron.com.ar/QUAD.HTM

Here are other methods: This one uses matrices, it like the continued fraction approach is excellent for computers.
http://fermatslasttheorem.blogspot.com/ … ution.html

The most famous pell equation is the cattle of the sun.

Last edited by bobbym (2009-07-26 03:24:11)

Hi Jane;

Here's how to use continued fractions to get the best rational approximations to the
roots of a quadratic.

one of the roots is phi ≈ 1.61803...

Now just replace the x in the fraction with the entire statement with 1).

Now again, replace x with 1).

Again.

You can truncate this at anytime to get the approximation.

And their is an algorithm for higher order polys too.

Last edited by bobbym (2009-07-27 18:08:34)

Hi Jane;

Great that you are posting what you are learning. Haven't done any CF's in 10 years. My favorite convergent to π is the one after 355/113, it gives 10 correct digits:

Last edited by bobbym (2009-07-28 06:22:22)