# Fibonacci to Avogadro: numbers with names

Of course, all numbers have *names* of a sort, in that they can be spelled with letters – the number 43 is ‘forty-three’, the number 1,000 is ‘one thousand’. But along with all of those words for numbers, there are several numbers that have more specific names. The most famous of these might just be ‘pi’, or π – the sixteenth letter of the Greek alphabet. The irrational number π (3.14159 to five decimal places) represents the value of the ratio of the circumference of a circle to its diameter. Let’s have a look at other numbers with names!

### 1. e

Pi became a significant mathematical constant, but it’s not the only alphabet that’s represented. You might recall using *e* in mathematics lessons. It’s ‘the transcendental number that is the base of Napierian or natural logarithms, approximately equal to 2.7182’. In case that doesn’t make everything clear, dive into a longer explanation.** **

### 2. prime number

A prime number is a number that cannot be divided by any whole number (without a remainder) other than itself and the number 1. The lowest prime number is therefore 2.

### 3. Graham’s number

Taking its name from Ronald Graham, the mathematician who introduced it, this number is mind-bogglingly large – even frighteningly large. Don’t believe me?

### 4. Fibonacci numbers

The numbers making up the Fibonacci sequence are 1, 1, 2, 3, 5, 8, and so on, where every number after the first is the sum of the two preceding numbers. The name *Fibonacci* comes from Leonardo Fibonacci, the Tuscan mathematician who discovered the series (and also popularized the use of the *Arabic numerals*; that is, 0-9).

### 5. Perfect number

A *perfect number* is one which is equal to the sum of its positive divisors (that is, its factors). For example, the first perfect number is 6. It can be divided by 1, 2, and 3, and these numbers add together to make 6. The only other perfect number under a hundred is 28 (adding together 1, 2, 4, 7, and 14), and there are only four under a million.

### 6. Mach number

This dimensionless number represents the ratio of the relative speed of a body and a fluid (especially air) to the local speed of sound in the fluid. Thus, Mach one is a supersonic speed referring to a Mach number of one. The name comes from Austrian physicist and philosopher Ernst Mach.

### 7. Pythagorean numbers

Named after Pythagoras, the noted Greek philosopher, Pythagorean numbers refer either to a number as conceived in cosmic or mythical terms by Pythagoras or his followers, or to the integers in a Pythagorean triple, corresponding to the sides of a right-angled triangle.

### 8. Avogadro’s number

Taking its name from Italian scientist Count Amedeo* Avogadro* (1776–1856), Avogadro’s number is the number of atoms or molecules in one mole of a substance, and is equal to 6.023 x 10^{23}. In this case, a mole is not your favorite burrowing mammal, but rather the SI unit of amount of substance that is equal to the quantity containing as many elementary units as there are atoms in 0.012 kg of carbon-12. Don’t you remember your chemistry?!

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