Finding Fibonacci Finding Fibonacci The Quest to Rediscover the Forgotten Mathematical genius Who changed the World KeiTh devlin Princeton University Press Princeton and Oxford Copyright © 2017 Keith Devlin Requests for permission to reproduce material from this work should be sent to Permissions, Princeton University Press Published by Princeton University Press, 41 William Street, Princeton, New Jersey 08540 In the United Kingdom: Princeton University Press, 6 Oxford Street, Woodstock, Oxfordshire OX20 1TR press.princeton.edu Jacket art courtesy of Shutterstock All Rights Reserved ISBN 978- 0- 691- 17486- 0 Library of Congress Control Number 2016962928 British Library Cataloging- in- Publication Data is available This book has been composed in Gentium Plus Printed on acid- free paper ∞ Printed in the United States of America 1 3 5 7 9 10 8 6 4 2 contents PRelude Sputnik and calculus 1 chaPTeR 1 The Flood Plain 5 chaPTeR 2 The Manuscript 18 chaPTeR 3 First Steps 35 chaPTeR 4 The Statue 42 chaPTeR 5 a Walk along the Pisan Riverbank 56 chaPTeR 6 a very boring book? 64 chaPTeR 7 Franci 72 vi contents chaPTeR 8 Publishing Fibonacci: From the cloister to amazon.com 85 chaPTeR 9 Translation 97 chaPTeR 10 Reading Fibonacci 116 chaPTeR 11 Manuscript hunting, Part i (Failures) 138 chaPTeR 12 Manuscript hunting, Part ii (Success at last) 151 chaPTeR 13 The Missing link 167 chaPTeR 14 This Will change the World 181 chaPTeR 15 leonardo and the birth of Modern Finance 192 chaPTeR 16 Reflections in a Medieval Mirror 213 aPPendix guide to the chapters of liber abbaci 228 bibliogRaPhy 236 index 239 Finding Fibonacci Prelude Sputnik and Calculus If my name is familiar to you, chances are it’s because you have read one of my mathematics books (I’ve written around 35, many of them for the general reader), or perhaps you’ve read an article I have published in a magazine or newspaper, read one of my blogposts (I maintain four blogs), taken part in one of the sessions of my massively open online course (MOOC) on mathematics, or heard me on National Public Radio, where I am known as “the Math Guy.” Yes, I am that Keith Devlin. And yes, I love math. By most people’s standards, I’m also pretty good at math. But it wasn’t always so. In the first few years of elementary school, I was one of the poorest performing children in mathematics. In fact, I was the last kid in the class to memorize my multiplication tables, a rite of passage that loomed large in the mathematics education mandated in 1950s England. Mastering the multiplication tables (today’s math educators call them “multiplicative number bonds,” presumably to give them a more modern ring) caused me so much anxiety that my parents had to go to the teacher to explain that