## Odd period square roots

Published on Friday, 27th February 2004, 06:00 pm; Solved by 18912;
Difficulty rating: 20%

### Problem 64

All square roots are periodic when written as continued fractions and can be written in the form:

$$\sqrt{N} = a_0 + \dfrac{1}{a_1 + \dfrac{1}{a_2 + \dfrac{1}{a_3 + ...}}}$$

For example, let us consider $\sqrt{N}$:

\begin{align} \sqrt{N} &= 4 + \sqrt{23} - 4\\ &= 4 + \dfrac{1}{\dfrac{1}{\sqrt{23} - 4}}\\ &= 4 + \dfrac{1}{\dfrac{\sqrt{23} +4}{7}}\\ &= 4 + \dfrac{1}{1 + \dfrac{\sqrt{23} - 3}{7}} \end{align}

If we continue we would get the following expansion:

$$\sqrt{23} = 4 + \dfrac{1}{1 + \dfrac{1}{3 + \dfrac{1}{1 + \dfrac{1}{8 + ...}}}}$$

The process can be summarised as follows:

\begin{align} a_0 = 4, \dfrac{1}{\sqrt{23} - 4} &= \dfrac{\sqrt{23} + 4}{7}\\ &= 1 + \dfrac{\sqrt{23} - 3}{7}\\ a_1 = 1, \dfrac{7}{\sqrt{23} - 3} &= \dfrac{7(\sqrt{23} + 3)}{14}\\ &= 3 + \dfrac{\sqrt{23} - 3}{2}\\ a_2 = 3, \dfrac{2}{\sqrt{23} - 3} &= \dfrac{2(\sqrt{23} + 3)}{14}\\ &= 1 + \dfrac{\sqrt{23} - 4}{7}\\ a_3 = 1, \dfrac{7}{\sqrt{23} - 4} &= \dfrac{7(\sqrt{23} + 4)}{7}\\ &= 8 + \sqrt{23} - 4\\ a_4 = 8, \dfrac{1}{\sqrt{23} - 4} &= \dfrac{\sqrt{23} + 4}{7}\\ &= 1 + \dfrac{\sqrt{23} - 3}{7}\\ a_5 = 1, \dfrac{7}{\sqrt{23} - 3} &= \dfrac{7(\sqrt{23} + 3)}{14}\\ &= 3 + \dfrac{\sqrt{23} - 3}{2}\\ a_6 = 3, \dfrac{2}{\sqrt{23} - 3} &= \dfrac{2(\sqrt{23} + 3)}{14}\\ &= 1 + \dfrac{\sqrt{23} - 4}{7}\\ a_7 = 1, \dfrac{7}{\sqrt{23} - 4} &= \dfrac{7(\sqrt{23} + 4)}{7}\\ &= 8 + \sqrt{23} - 4\\ \end{align}

It can be seen that the sequence is repeating. For conciseness, we use the notation $\sqrt{23} = [4; (1, 3, 1, 8)]$, to indicate that the block (1,3,1,8) repeats indefinitely.

The first ten continued fraction representations of (irrational) square roots are:

$\sqrt{2} = [1; (2)]$, period = 1
$\sqrt{3} = [1; (1, 2)]$, period = 2
$\sqrt{5} = [2; (4)]$, period = 1
$\sqrt{6} = [2; (2, 4)]$, period = 2
$\sqrt{7} = [2; (1, 1, 1, 4)]$, period = 4
$\sqrt{8} = [2; (1, 4)]$, period = 2
$\sqrt{10} = [3; (6)]$, period = 1
$\sqrt{11} = [3; (3, 6)]$, period = 2
$\sqrt{12} = [3; (2, 6)]$, period = 2
$\sqrt{13} = [3; (1, 1, 1, 1, 6)]$, period = 5

Exactly four continued fractions, for $N \le 13$, have an odd period.

How many continued fractions for $N \le 10000$ have an odd period?