# Keith Devlin - The Man of Numbers

**Keith Devlin** 2011. *The Man of Numbers. Fibonacci’s arithmetic revolution*. New York: Walker and Company, 183 pp.

This little book should be a lot more interesting that it turned out to be. The author divulges the reason for this early on: he likens the subject of the story, **Leonardo of Pisa**, more often known as Fibonacci, to **Steve Jobs** or **Bill Gates**. All three had transformative roles, not for original ideas, but for making the ideas of other people into technology that everyone can use easily. The story is about how **Leonardo** learned Arabic (actually Hindu- or Indian-Arabic) numerals and how to do arithmetic with them while travelling with his father in North Africa. **Leonardo** transformed calculations and trade in Europe by writing an instruction book, *Liber abacci*, so that others could do it. Arguably he wrote the first software manual that now exists only as 3 copies of the 2nd edition. It is about as interesting as a software manual: a 600+ page book mostly comprising worked examples (no screen-shots but instead images of hand shapes as number prompts). **Leonardo** was made famous and wealthy by his book, apparently in much the same way that **Jobs** and **Gates** *et al.* are now.

**Keith Devlin** is a retired mathematician with many other books published, but he struggles a bit with the content here, for the above reason but also because little is known of the life of **Leonardo** so the book has many statements qualified with “probably”, “could have”, “likely” and so on.

There is always something to learn and be interested in, however. *Liber abacci* was first published in 1202 but Arabic number notation was brought to Europe much earlier, when they invaded Spain in 711 CE. The oldest known Latin manuscript documenting Arabic numbers, *Codex Vigilanus* was written in 976, more than 200 years before the birth of **Leonardo**. **Leonardo** died sometime after about 1241, and within a few decades Pisa (whose port became useless due to siltation of the Arno) was defeated by Genoa, which along with Florence became the major powers in that part of what is now Italy.

The now-famous Fibonacci series (in **Leonardo**’s example a sequence of numbers generated by iterations of breeding rabbits, starting with a single pair) was invented by Indian mathematicians in antiquity but used by **Leonardo** in *Liber abacci* as one of many examples of the utility of Arabic numerals. The Fibonacci series is widespread in nature (not only rabbits generate the sequence) but the nickname Fibonacci was attributed to **Leonardo** by a French historian 700+ years later, in 1838. **Keith Devlin** doesn’t explain the etymology which I guess is unknown?

The bibliography has many original sources, plus a couple of recent books that seem worth a look:
**Brian Butterworth** (1999) *What Counts: How every brain is hardwired for math* and **Keith Devlin** (2000) *The Math Gene: How mathematical thinking evolved and why numbers are like gossip*.