# Finding the Golden Ratio

I recently was exploring the Golden Ratio. I was really surprised what a simple (and recursive) relationship it comes from: Starting with this this image, here is the derivation of the golden ratio...

From this relationship we see in this image, we can derive this equation: $\frac{a}{b} = \frac{a+b}{a}$

And if we then let b=1 (so we can find the ratio of a to 1), we get: $a = \frac{a+1}{a}$

Then, with this formula, we can define a function: $f(a) = \frac{a+1}{a}$

If we then find the fixed-point in this function (so that f(x) = x), then we will have found a solution to $a = \frac{a+1}{a}$

A simple way to think about the fixed-point of this function is this: it is the place where $y=x$ (or $f(x) = x$ ) intersects with our function. And that intersction point should be the golden ratio.

In :
# Plot y=(x+1)/x and y=x
x=numpy.linspace(0.5,3,1000)
y=map(lambda a: (a+1.0)/a, x)
y2=map(lambda a: a, x)
plot(x, y)
plot(x, y2, 'r')

# Make graph square
fig = matplotlib.pyplot.gcf()
fig.set_size_inches(5,5) Another way to find the golden ratio is to use Fixed-point iteration - again, since we know that finding the fixed point of the function:

$f(a) = \frac{a+1}{a}$

is the golden ratio.

In :
golden_ratio = reduce(lambda acc,_: (acc+1.0)/acc, xrange(100), 1)
print golden_ratio

1.61803398875