# Phi

**Phi** (in lowercase, either or ; capitalized, ) is the 21st letter in the Greek alphabet. It is used frequently in mathematical writing, often to represent the constant . (The Greek letter Tau () was also used for this purpose in pre-Renaissance times.)

## Contents

## Use

appears in a variety of different mathematical contexts: it is the limit of the ratio of successive terms of the Fibonacci sequence, as well as the positive solution of the quadratic equation .

is also equal to the continued fraction and the continued radical . It is the only positive real number that is one more than its square and one less than its reciprocal.

It is also where is the nth number in the Fibonacci sequence. In other words, if you divide the ^{th} term of the Fibonacci series over the ^{th} term, the result approaches as increases.

## Golden ratio

is also known as the Golden Ratio. It was commonly believed by the Greeks to be the most aesthetically pleasing ratio between side lengths in a rectangle. The Golden Rectangle is a rectangle with side lengths of 1 and ; it has a number of interesting properties.

The first fifteen digits of in decimal representation are

## Other Usages

- is also commonly used to represent Euler's totient function.