Phi

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Phi (from the Greek letter φ) is an irrational number often referred to as "The Golden Ratio". It can be found in many places in nature.

Definition
Take a line segment, divided into two parts: a and b. When the ratio between the whole segment (a+b) and a is equal to the ratio between a and b, this ratio is equal to phi.

Properties

 * Phi is an irrational number.
 * 1 / phi = phi - 1.
 * The ratio between two successive fibonacci numbers is close to phi.