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## Plus Advent Calendar Door #21: The Plus top 5

Apart from the Olympic Games and Alan Turing's centenary, 2012 will see another important event: the 15th birthday of *Plus*! To get into party mood, here are the 5 most popular *Plus* articles since current records began. A big thank you to the wonderful and generous people who have contributed these and all other *Plus* articles!

Number 1: Adam Smith and the invisible hand

Adam Smith is often thought of as the father of modern economics. In his book *An Inquiry into the Nature and Causes of the Wealth of Nations* Smith decribed the "invisible hand" mechanism by which he felt economic society operated. Modern game theory has much to add to Smith's description.

Number 2: Mysterious number 6174

6174 is a very mysterious number. This article explains why, and how beautiful mathematical oddities can inspire us to discover new mathematics.

Number 3: Why is the violin so hard to play?

Number 3: As anyone starting out knows, the violin is a difficult instrument. It takes time before the novice player can expect to produce a musical note at the desired pitch, instead of a whistle, screech or graunch. This article explains why.

Number 4: The life and numbers of Fibonacci

Fibonacci not only gave us the famous number sequence but also introduced the decimal system to Europe.

Number 5: 101 uses of the quadratic equation

It isn't often that a mathematical equation makes the national press, far less popular radio, or most astonishingly of all, is the subject of a debate in the UK parliament. However, as this article describes, in 2003 the good old quadratic equation, which we all learned about in school, reached these dizzy pinnacles of fame.