Many Mathematics teachers will know the story of the murder of Hippasus of Metapontum around 500BCE for demonstrating that root 2 is irrational. The existence of such numbers challenged the Pythagorean world view that all was number (by which they meant measurable).
The recognition of different types of numbers has been an integral part of the development of our number system. Whilst in China negative numbers were recognised as early as 100BCE, Diophantus considered them absurd. Zero has been accepted in some form since at least 2000 BCE – though usually as a space or symbol just indicating ‘no number’. Brahmagupta, born in India 598 CE, was the first to treat 0 as an actual number. The word “zero” comes from the Arabic “sifr”. The Persian mathematician, Al-Khwārizmī, wrote a treatise in 825 on Hindu numerals which led, eventually, to their adoption in Europe with Fibonnacci’s help. “Sifr” is also the origin of the word “cipher”. Some think that there is a link between zero and codes as the Hindu-Arabic numbers originally had to be used secretly. The city of Florence banned their use in 1299. Perhaps, the concern was that the populace would find arithmetic too easy and this would erode the power of the educated elites.
Individual numbers have also been banned. From Roman times, a 6 x 6 magic square, using the numbers 1 to 36, was used as a talisman. The Church banned the possession of this magic square. The overall sum is 666 – the ‘number of the beast’ or the ‘anti-christ’. Fear of 666 has a name: Hexakosioihexekontahexaphobia. US President Ronald Reagan had their house number in Los Angeles changed from 666 to 668.
In China, it is illegal to use 8964 as a pin code since it is the date of the Tiananmen Square massacre (4 June 1989). As recently January this year, Sony prosecuted George Holtz for showing how to by-pass Play Station security. Part of the lawsuit was that he published copyrighted numbers. It was illegal to know these numbers unless authorised. This is a strange idea – that someone can own a number!