## Summing Fibonacci numbers

In this recipe, we will sum the even-valued terms in the Fibonacci sequence whose values do not exceed four million. The Fibonacci series is a sequence of integers starting with zero, where each number is the sum of the previous two; except, of course, the first two numbers zero and one.

### Note

For more information, read the Wikipedia article about Fibonacci numbers at http://en.wikipedia.org/wiki/Fibonacci_number .

This recipe uses a formula based on the golden ratio, which is an irrational number with special properties comparable to pi. It we will use the
`sqrt`

,
`log`

,
`arange`

,
`astype`

, and
`sum`

functions.

### How to do it...

The first thing to do is calculate the golden ratio (http://en.wikipedia.org/wiki/Golden_ratio), also called the golden section or golden mean.

Calculate the golden ratio.

We will be using the

`sqrt`

function to calculate the square root of five:phi = (1 + numpy.sqrt(5))/2 print "Phi", phi

This prints the golden mean:

**Phi 1.61803398875**Find the index below...