**Lived c. 210 – 295 AD**

Diophantus is known as the father of algebra. Roughly five centuries after Euclid’s era, he solved hundreds of algebraic equations in his great work *Arithmetica*, and was the first person to use algebraic notation and symbolism.

Today we usually indicate the unknown quantity in algebraic equations with the letter x. In the oldest copies of *Arithmetica* the unknown quantity is indicated by a character similar to an accented Greek letter sigma: ς’.

*Arithmetica* inspired some of the world’s greatest mathematicians including Leonhard Euler and Pierre de Fermat to make significant new discoveries.

### The Life of Diophantus

Diophantus (pronounced dy-o-Fant-us) flourished during the third century AD in the Greco-Roman city of Alexandria in Egypt.

Like other educated people in the Eastern Mediterranean at that time he was a Greek speaker. We do not know what he looked like. The years of his birth and death are highly uncertain.

The little we know about Diophantus’ life comes from a word puzzle reputed to be his epitaph. Presumably it was written by a friend who knew Diophantus’ life story and who wished to give him a fittingly algebraic memorial. The epitaph is known to us through the Greek author Metrodorus who recorded it in his anthology of puzzles in about the sixth century.

A 1941 translation of the epitaph by Ivor Thomas says:

This tomb holds Diophantus. Ah, what a marvel.

And the tomb tells scientifically the measure of his life.

God vouchsafed that he should be a boy for the sixth part of his life;

When a twelfth was added, his cheeks acquired a beard;

He kindled for him the light of marriage after a seventh,

And in the fifth year after his marriage He granted him a son.

Alas! late-begotten and miserable child, when he had reached the measure of half his father’s life, the chill grave took him.

After consoling his grief by this science of numbers for four years, he reached the end of his life.

We can solve the epitaph as an algebraic equation:

And we find x = 84, from which it follows:

Diophantus’ boyhood lasted 14 years.

When he was 21, his beard grew.

He married at 33.

His son was born when Diophantus was 38.

His son died age 42, when Diophantus was 80.

Diophantus died at age 84.

#### Lifetimes of Selected Greek Mathematicians

### A Brief History of Algebra before Diophantus

Algebra has a long history. Between 2000 – 1600 BC the Babylonians produced rather sophisticated algebra some of which survives on clay tablets. Babylon’s mathematicians were not concerned with exact numerical solutions to problems – they were happy with good approximations from reference tables they compiled. They could solve quadratic equations using geometric drawings of areas and lengths of squares.

The *Rhind Mathematical Papyrus* dating to about 1550 BC contains Ancient Egyptian algebra, such as: What must the number (1 + ½ + ¼) be multiplied by to give the answer 10?

The *Nine Chapters on the Mathematical Art* from China was composed in stages over a timespan possibly stretching between 1000 BC – 200 AD. In the eighth chapter, agricultural problems give rise to linear equations solved using rows of numbers similar to matrices.

Modern historians of mathematics sometimes wrangle over Book 2 of Euclid’s Elements from about 300 BC, debating whether it contains algebra written in geometric language. Certainly the great 11th century Persian mathematician Omar Khayyam had no doubts:

*Elements*.”

Khayyam was arguing for algebra to be seen as a legitimate branch of mathematics. Many mathematicians felt uneasy about algebra because it lacked the compelling logical rigor Euclid’s *Elements* had brought to geometry. Khayyam’s work with cubic equations made him certain that algebra and geometry were linked.

## The Mathematics of Diophantus

### Introduction to *Arithmetica*

Diophantus tells us at the beginning of his classic work *Arithmetica* that he has written it as a textbook to help his friend Dionysius (and others presumably) to solve mathematics problems. *Arithmetica* tackles the construction and solution of equations to find one or more unknowns. All copies of *Arithmetica* in Diophantus’ time were handwritten. Copies were made by scribes for over a thousand years until the first copies were printed in Europe.

“Perhaps the subject will appear rather difficult, because is not yet familiar (beginners are, as a rule, too ready to despair of success); but you, with the impulse of your enthusiasm and the benefit of my teaching, will find it easy to master; for eagerness to learn, when seconded by instruction, ensures rapid progress.”

*Arithmetica*, c. 250 AD

*Arithmetica* may have been conceived in a similar way to Euclid’s *Elements*. Euclid compiled and, where necessary, improved on the work of mathematicians such as Eudoxus and the Pythagoreans. Any other books dating from classical times with similar themes to *Arithmetica* have been lost.

### Volumes of *Arithmetica*

Diophantus composed *Arithmetica* in thirteen volumes of which six survived in Greek. Four exist as Arabic translations.

- Volumes 1, 2, and 3 survive in Greek from Byzantium.
- Volumes 4, 5, 6, and 7 exist as Arabic translations of Greek from Baghdad. They were translated in the ninth century by the Byzantine Christian scholar Kostas Luka known in Arabic as Qusta ibn Luqa. The books were discovered in 1971 in Meshed, Iran, where they had been misfiled for centuries in the Astan Quds Library as the work of Qusta ibn Luqa rather than Diophantus.
- Three other volumes of
*Arithmetica*exist in Greek, but their volume numbers are uncertain – they could be any three of volumes 8, 9, 10, 11, 12, or 13.

The Arabic translations contain more commentary on the solutions than the Greek versions. It is possible that the Arabic editions were copied from the lost edition adapted by Hypatia for the students at her school.

### The Mathematics in *Arithmetica*

Diophantus begins with definitions and rules. For example, he defines the results of multiplication of quantities with different signs and tells his readers he will indicate subtraction with a symbol. He says:

*Arithmetica*, c. 250 AD

### Book 1, Problem 1

**The Problem**

In the very first problem in the very first book of *Arithmetica* Diophantus asks his readers to divide a given number into two numbers that have a given difference.

The number he gives his readers is 100 and the given difference is 40.

**The Solution**

Diophantus writes [we use modern notation]:

2x + 40 = 100

Hence x = 30

Therefore the two numbers are 30 and 70.

### Things Get Harder

Diophantus continues in this way, describing hundreds of problems which he translates into solvable equations. The level of difficulty rises as he introduces quadratics, cubics, and equations in higher powers of x.

### Quadratics, Cubics, and More

At high school, we learn about equations of general form ax^{2} + bx + c = 0; these are quadratic equations.

Cubic equations are of general form ax^{3} + bx^{2} + cx + d = 0. Cubic equations are harder to solve than quadratics.

Diophantus ignores negative and irrational solutions to equations.

A major criticism of Diophantus is that he rarely offers his readers general methods to solve a particular class of problem such as quadratics.

The level of mathematical sophistication in *Arithmetica* is sometimes disconcertingly high. It required the genius of 18th century mathematician Joseph Lagrange to finally prove that every number can be written as the sum of four squares – a result Diophantus was well aware of.

### Diophantine Equations

In *Arithmetica*, Diophantus launched the study of indeterminate equations – these are polynomial equations in which the number of unknowns exceeds the number of equations given.

The solutions to Diophantus’ indeterminate equations were always positive rational numbers. (Diophantus was interested only in single number solutions, so he did not, for example, seek two numbers as solutions to quadratic equations.)

Nowadays we define a Diophantine Equation as an indeterminate equation whose solutions must be integers.

Consider this Diophantine Equation:

If n=2, we have Pythagoras’s theorem, which has an infinite number of whole number solutions, the most famous example of which is the 3-4-5 triangle: x=3, y=4, z=5.

Fermat’s Last Theorem claims that if n is a whole number bigger than 2, the equation has no whole number solutions for x, y and z. Fermat claimed to have proved this for all values of n, but famously said the margin of his book was too small to write his proof. Not surprisingly, the book was Diophantus’ *Arithmetica*.

### After *Arithmetica*

About four centuries after Diophantus wrote *Arithmetica*, the great seventh century Indian mathematician Brahmagupta found the general solution for linear and quadratic equations.

Qusta ibn Luqa’s ninth century translation of *Arithmetica* into Arabic followed soon after the great Persian mathematician al-Khwarizmi presented systematic solutions of linear and quadratic equations.

Brahmagupta and al-Khwarizmi’s works were less ambitious than Diophantus’ in that they dealt with equations in x and x^{2}. Diophantus frequently dealt with cubic and higher power equations, up to x^{9}. Brahmagupta and al-Khwarizmi’s crucial contribution is the concept of the *general solution* of an equation. General solutions are not offered in any of the surviving books of *Arithmetica*.

The fusion of al-Khwarizmi’s highly systematic algebra with Diophantus’ intriguing problems and solutions led to a great flowering of algebra in Persia and the Islamic world. In the eleventh century, Omar Khayyam showed how the intersections of conic sections such as parabolas and circles can yield geometric solutions of cubic equations.

Following the Renaissance, European mathematicians of the highest rank were captivated by the mathematics of *Arithmetica*.

In 1535, Niccolo Tartaglia found general solutions for all cubic equations.

In the 1600s, *Arithmetica* inspired a great number of Pierre de Fermat’s new ideas. He worked on *Arithmetica* for pleasure, much as a modern person might work on a crossword puzzle or a game of Sudoku. When new ideas came to him, he scribbled them in the margin of the book. These ideas, including Fermat’s Last Theorem, transformed number theory.

In the 1700s, about 1,500 years after Diophantus wrote *Arithmetica* Leonhard Euler took great delight and inspiration from attacking its trickier problems. Euler’s words provide us with a fitting final tribute to Diophantus:

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**Further Reading**

Sir Thomas Heath

Diophantus of Alexandria; A study in the history of Greek algebra

Cambridge University Press, 1910

Morris Kline

Mathematical Thought from Ancient to Modern Times

Oxford University Press, New York, 1972

Norbert Schappacher

Diophantus of Alexandria : a Text and its History