Euclid's Algorithm and Musical Intervals
Age 16 to 18
Challenge Level
How many major thirds are there in an octave on a musical scale?
Going back in history, before the discovery of logarithms, some
theorists used Euclid's algorithm to find the answer to this
question.
A rational approximation ${m\over n}$ for the relationship between
5/4, the musical interval called the major third, and the octave
2/1, is given by $$ \left({5\over 4}\right)^m \approx \left({2\over
1}\right)^n, $$ where $m$ and $n$ are integers. Using Euclid's
algorithm show that ${m\over n}={28\over 9}$ gives a first
approximation and find three closer rational
approximations.
[See also the problems
Tuning and Ratio and
Rarity. The set of three problems on mathematics and music was
devised by Benjamin Wardaugh who used to be a member of the NRICH
team. Benjamin is now doing research into the history of
mathematics and music at Oxford University and his article
Music
and Euclid's Algorithm should help you with this
problem.]