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SCOTT SIMON, Host:

This is WEEKEND EDITION from NPR News. I'm Scott Simon.

Join us now on a trip back to the early 13th century, in the land we now call Italy, to hear the numerous tales of a man called Leonardo, enshrined in history as Fibonacci and his "Liber Abaci."

Huh? Leonardo da Vinci told a fib about Liberace? Okay, Take 2. the year 1202, a young mathematician named Leonardo da Pisa, no relation to da Vinci, published a book titled "Liber Abaci." That's Latin for "Book of Calculation," which introduced practical uses for the Hindu-Arabic numerals zero through nine to Western Europe for the very first time.

The book revolutionized commerce, banking, science and technology, and established the basis of modern arithmetic, algebra and other disciplines. And we do mean disciplines.

SIMON: Fibonacci's Arithmetic Revolution." Keith joins us from the studios at Stanford University, where he teaches.

Keith, thanks so much for being with us.

KEITH DEVILIN: Thanks, Scott. Good to be here again.

SIMON: This book sold even more copies than the monk's book of diet and exercise, didn't it?

(SOUNDBITE OF LAUGHTER)

DEVILIN: Actually though. And this is kind of interesting. One of the things I like about Leonardo, especially living here in the middle of Silicon Valley, is he knew how to market something. Having written this really big, incredible book, he realized that that book was probably too difficult for most of the people he was trying to reach, which were the traders and commercial people in Pisa, which at the time was the center of the Western commercial world.

He ended up writing a smaller, easier to read version of that book. And in many ways it was a combination of the big scholastic book and the smaller easier to read version had freely as is that really causes one was in fact the world's first revolution.

SIMON: You compare him to some major figures today.

DEVILIN: This is Steve Jobs, Bill Gates. It's the computer revolution that we lived through in the 1980s and the parallels are actually uncanny.

SIMON: Well, take them up for us. 'Cause as I understand it, you know, zero to nine numerals had been around in Hindu and Arabic cultures for centuries.

DEVILIN: The numerals were around and sort of people knew that you could represent numbers with them. But that wasn't how people did business. What they did was they recorded everything in good old Roman Numerals. And if they wanted calculations, they went down the street to someone who was adept at using a physical abacus. It was actually a board on which you moved - with lines ruled on it on which you moved pebbles around. But it was a very crude and inefficient way of doing business.

What Leonardo does, and he when seized this method and writes this book "Libre Abaci," which is really the first practical arithmetic textbook in the Western world, within a few decades of "Libre Abaci" appearing, you've got what may have been a thousand or more different people writing practical arithmetic textbooks. What was happening, the ordinary person who wanted to set up a business, and didn't have a lot of money to pay people to do the accounting for them, could do it for themselves.

If you look at the beginnings of accounting, doubly entry bookkeeping, insurance, banking, all of that commercial stuff, it all came out of Pisa in the 13th century. And the key for that to happen was that people could do arithmetic efficiently for the first time in history.

SIMON: So why is Leonardo da Pisa referred to as Fibonacci?

DEVILIN: He was given that name by a historian. The name is actually comes from the Latin phrase: filius Bonacci. In the beginning of Leonardo's book, he says this is the book of calculation written by the son of Bonacci, filius Bonacci. He uses that phrase in the first opening sentence. And so, this historian in the 19th century takes up for his filius Bonacci, simplifies it to Bonacci. And for reasons, best known to himself, gives Leonardo that nickname.

SIMON: If "Libre Abaci" hadn't been written, would trading anything more than a couple of sacks of grain be possible?

DEVILIN: It would have happened, sure. I mean one of the things about almost all of mathematics is that it'll eventually surface and get used. It's a matter of who does it and when. And this is another of those similarities between Leonardo and Bill Gates and Steve Jobs and the people in the 1980s. And what Jobs did was simply take something that other people had developed and present it in a very easy to use way; and essentially turned computing into a consumer product. That's what Leonardo did.

SIMON: Keith Devlin, our Math Guy, his new book, "The Man of Numbers: Fibonacci's Arithmetic Revolution," hit the bookstores this week. And you can read an excerpt on our web site at NPR.org.

Thanks Keith.

DEVILIN: Oh, my pleasure, Scott. As usual.

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