## Walking the Tree.

It’s just fun playing around with Stern-Brocot trees and a bit of Haskell…! For any fraction on the Stern-Brocot tree there is just one path to it from the root. That path can written as a sequence of left or right ‘turns’ at each node depending on whether the target node is smaller or larger than the current node. This leads to the Stern Path For example in this small tree the path to, say, 4/5 is, from 1/1, LRRR. The fraction 5/2 has path RRL. To get to 5/8 we go LRLR… and so on. (If you want to Read More

## Ford and his Circles.

A Ford circle is a circle derived from a pair of numbers that are co-prime, i.e. they have no common factors. For a pair of co-prime integers p and q the Ford circle has radius r and centre at a point P(x, y) where r = 1/(2q^2) and P = (p/q, r) No matter what co-prime numbers, p and q, are used to create Ford circles the circles never intersect and they are all tangential to the horizontal axis. Now, we could generate ‘random’ Ford circles by picking any old co-prime pair (p, q). However, the Farey Read More

## Count the Fractions.

This is the first in a series of posts about sequences of fractions, circles, trees of fractions, binary search trees and ways of representing rational numbers as paths through these trees . The idea is to enjoy a bit of recreational mathematics and to use Haskell to express some of the notions in its own succinct way. One Lot Of Fractions First we’ll look at all unique and simplified fractions between 0 and 1 with a denominator no larger than a given value. For example, with a maximum denominator of 3, 4 and then 5 we have     For Read More

## Squares and Graphs

Recently I came across this intriguing little puzzle… Take the integers 1..n and, if possible, arrange them in a list in such a way that consecutive numbers in the list add up to a square number. Use each number 1..n once only. In exploring this puzzle I started writing down the numbers and forming a graph where two numbers are connected if they add up to a square. Drawing such graphs is fun but slightly tedious for, for example, [1..300]… Here’s such a graph for numbers 1..15. Starting at 8 then 1 etc. and following the path gives the solution Read More

## Continued Fractions, Pell’s Equation and Scary Numbers.

This is Pell’s equation: where n is a positive integer that isn’t a perfect square. Only integer solutions for x and y are sought and if n is not a perfect square then there are infinitely many integer solutions. It can be shown that the convergents of the continued fraction (CF) for the square root of n contains a solution known as the Fundamental Solution (FS). In practice this fundamental solution is the first convergent that satisfies the equation under consideration. Once this solution is known then all other solutions can be calculated from a simple recurrence relationship. i.e. If Read More

## More Continued Fractions Continued.

In the post Parsers to Fractions to Square Roots! we looked at continued fractions (CFs) expressed in list format and used that format to calculate the square root of integers. The technique didn’t really explore an effective way of generating the list form for a CF. In this short post we’ll look at how to calculate the CF list format for the square root of an integer. Then, from the list, calculate the convergents, each of which gives a better and better value for the square root. (The details of convergents have been covered in Continued Fractions Continued.) With a Read More

## The expressiveness of Haskell – Split a List.

Just recently playing around with one of the excellent Advent of Code problems (specifically 2015 question 6 ) a possible solution involved splitting a list into two parts where the first element goes into the first part, the second element into the second part and so on. For example [1, 2, 3, 4, 5, 6] -> [1, 3, 5], [2, 4, 6] So, in terms of function signatures we can write splitList :: [a] -> ([a], [a]) Notice there’s no constraint on the type in the list, it’s a very general function. One way of doing this is is to make use of pattern matching Read More

## Synacor Virtual Machine.

Just recently I came across the ‘Synacor VM’ challenge. The problem is an interesting exercise in implementing a virtual machine using a supplied architecture definition and then running the VM using the given binary file as input. When run the program is an adventure type game which many people have completed but I have, so far, resisted that temptation. So here’s the spec.

I had a couple of attempts at this – in Haskell – and the main thing I learnt from this was… Yes, the Haskell type system is very expressive and allows for a sophisticated modelling Read More

## Continued Fractions Continued.

It seems to me that continued fractions (CFs) are perhaps too advanced for ‘A’ levels and too elementary for a degree maths course and are perhaps undervalued or ignored in schools and universities? Since my last post about fractions I’ve looked a little more at CFs and found they have applications ranging from factorising large numbers to gear ratio calculations. And they’re really interesting when their layers are peeled away with a bit of Haskell. So, a bit of playing with numbers and a bit of Haskelling- what’s not to like? Let’s start with a fraction, any fraction – say Read More

## Parsers to Fractions to Square Roots!

The earlier post Fractions to Phi to Fibonacci! showed a simple structure for Continued Fractions (CF) and used a CF representation of the Golden Ratio to derive the terms of the Fibonacci Sequence. It also alluded to a particular notation to express a CF and here we will describe that notation, create a parser for it and calculate the square roots of some integers from their specific CFs and finally derive a general CF that can be used to find the square root of any integer.   CF List Notation Essentially the CF is written as a list of ints Read More