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The Most Irrational Number
The most irrational numberThe most irrational number turns out to be a number already well known in geometry. It is the number
g = ( which is the length of the diagonal in a regular pentagon of side length 1. This number, known as the "golden mean," has played a large role in mathematical aesthetics. It is not clear whether its supreme irrationality has anything to do with its artistic applications. The golden mean satisfies the equation x2 - x - 1 = 0, so its continued fraction expansion is the simplest of all: g = $1 + \frac{1}{1+\frac{1}{1+\frac{1}{1+\ldots}}}$ Its convergents are 1, 2, 3/2, 5/3, 8/5, ... , the ratios of consecutive Fibonacci numbers. How well are these convergents approximating g? Here are the first few E/M ratios:
... Hurwitz' Theorem guarantees the existence of infinitely many convergents with E/M < 1. In this case the odd-numbered convergents must be discarded, and the even-numbered ones are getting as bad as they can be. (In fact this table is evidence that the factor $\sqrt{5}$ in Hurwitz' theorem cannot be improved!) So the golden mean can never have a rational approximation as good as 22/7 was for
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+ 1)/2 = 1.618033...
or even as good as 7/5 was for 
.
