In probability, a system of counting occurs frequently which involves a series of multiplications of the form:

$$n! = n ⋅ (n - 1) ⋅ (n - 2) ⋅ (n - 3) .... 2 ⋅ 1$$This function has the name factorial, and has the symbol '!'.

For example, 6! = 6 ⋅ 5 ⋅ 4 ⋅ 3 ⋅ 2 ⋅ 1 ( = 720)

1! = 1, and 0! = 1 (careful: 0! is ** NOT** equal to zero)

Factorials can be divided by other factorials: ${6!}/{4!} = {6 ⋅ 5 ⋅ 4 ⋅ 3 ⋅ 2 ⋅ 1}/{4 ⋅ 3 ⋅ 2 ⋅ 1} = {6 ⋅ 5} = 30$

A permutation of a group of symbols is this group in any arrangement in a definite order.

e.g the numbers 1, 2, 3 may be organised in the orders: 123, 132, 213, 231, 312, 321.

There are 6 (3!) possible permutations for 3 symbols. There are 24 (4!) possible permutations for 4 symbols, and 120 possible permutations for 5 (5!) symbols.

Rule: n symbols may be arranged n! ways.

The number of ways 5 items may be arranged, taking 3 at a time is: ${5!}/{2!} = 5 ⋅ 4 = 20$

Rule: n symbols taken r at a time may be arranged in ${n!}/{(n - r)!}$ ways.

6 cards taken two at a time form ${n!}/{(n - r)!} = {6!}/{(6 - 2)!} = {6!}/{4!} = 6 ⋅ 5 = 30$ pairs, where the order is important: (1,2) is different to (2,1).

This can be written as:

$$_nP_r = {n!}/{(n - r)!}$$In $_nP_r$ the order of the sub-set of r items is important. In many cases, the order is not relevant. For example, in dealing hands of cards, the player can rearrange the cards he is dealt.

Therefore, if the order of the sub-set is not important, there are fewer combinations possible. This quantity is given the symbol C.

$$_nC_r = {n!}/{{(n - r)!}r!}$$Content © Renewable.Media. All rights reserved. Created : January 1, 2015

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Mathematics is the most important tool of science. The quest to understand the world and the universe using mathematics is as old as civilisation, and has led to the science and technology of today. Learn about the techniques and history of mathematics on ScienceLibrary.info.

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Sydney Brenner, born 1927, is a South African molecular biologist. He is a leading figure in biology, having won the Nobel Prize, and authored or co-authored numerous seminal papers on fields related to genome and protein sequencing.

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"I knew Descartes," said Isaac Barrow. "René', I used to say. 'Have you got des cartes?' Then after a few tankards, he would say: 'Don't try to cheat me, Wheelie-boy. Cogito ergo sum...' - that's classical pidgin for 'I can think so I can add up as well as the next man'."

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