number theory

Number theory

Although Euclid handed down a precedent for number theory in Books VII–IX of the Elements, later writers made no further effort to extend the field of theoretical arithmetic in his demonstrative manner. Beginning with Nicomachus of Gerasa (flourished c. 100 CE), several writers produced collections expounding a much simpler form of number theory. A favourite result is the representation of arithmetic progressions in the form of “polygonal numbers.” For instance, if the numbers 1, 2, 3, 4,…are added successively, the “triangular” numbers 1, 3, 6, 10,…are obtained; similarly, the odd numbers 1, 3, 5, 7,…sum to the “square” numbers 1, 4, 9, 16,…, while the sequence 1, 4, 7, 10,…, with a constant difference of 3, sums to the “pentagonal” numbers 1, 5, 12, 22,…. In general, these results can be expressed in the form of geometric shapes formed by lining up dots in the appropriate two-dimensional configurations (see figure). In the ancient arithmetics such results are invariably presented as particular cases, without any general notational method or general proof. The writers in this tradition are called neo-Pythagoreans, since they viewed themselves as continuing the Pythagorean school of the 5th century BCE, and, in the spirit of ancient Pythagoreanism, they tied their numerical interests to a philosophical theory that was an amalgam of Platonic metaphysical and theological doctrines. With its exponent Iamblichus of Chalcis (4th century CE), neo-Pythagoreans became a prominent part of the revival of pagan religion in opposition to Christianity in late antiquity.

Polygonal numbersThe ancient Greeks generally thought of numbers in concrete terms, particularly as measurements and geometric dimensions. Thus, they often arranged pebbles in various patterns to discern arithmetical, as well as mystical, relationships between numbers. A few such patterns are indicated in the figure

An interesting concept of this school of thought, which Iamblichus attributes to Pythagoras himself, is that of “amicable numbers”: two numbers are amicable if each is equal to the sum of the proper divisors of the other (for example, 220 and 284). Attributing virtues such as friendship and justice to numbers was characteristic of the Pythagoreans at all times.

Of much greater mathematical significance is the arithmetic work of Diophantus of Alexandria (c. 3rd century CE). His writing, the Arithmetica, originally in 13 books (six survive in Greek, another four in medieval Arabic translation), sets out hundreds of arithmetic problems with their solutions. For example, Book II, problem 8, seeks to express a given square number as the sum of two square numbers (here and throughout, the “numbers” are rational). Like those of the neo-Pythagoreans, his treatments are always of particular cases rather than general solutions; thus, in this problem the given number is taken to be 16, and the solutions worked out are 256/25 and 144/25. In this example, as is often the case, the solutions are not unique; indeed, in the very next problem Diophantus shows how a number given as the sum of two squares (e.g., 13 = 4 + 9) can be expressed differently as the sum of two other squares (for example, 13 = 324/25 + 1/25).

To find his solutions, Diophantus adopted an arithmetic form of the method of analysis. He first reformulated the problem in terms of one of the unknowns, and he then manipulated it as if it were known until an explicit value for the unknown emerged. He even adopted an abbreviated notational scheme to facilitate such operations, where, for example, the unknown is symbolized by a figure somewhat resembling the Roman letter S. (This is a standard abbreviation for the word numberin ancient Greek manuscripts.) Thus, in the first problem discussed above, if S is one of the unknown solutions, then 16 − S2 is a square; supposing the other unknown to be 2S − 4 (where the 2 is arbitrary but the 4 chosen because it is the square root of the given number 16), Diophantus found from summing the two unknowns ([2S − 4]2 and S2) that 4S2 − 16S + 16 + S2 = 16, or 5S2 = 16S; that is, S = 16/5. So one solution is S2 = 256/25, while the other solution is 16 − S2, or 144/25.

greek trignometry and mensuration

Greek trigonometry and mensuration

After the 3rd century BCE, mathematical research shifted increasingly away from the pure forms of constructive geometry toward areas related to the applied disciplines, in particular to astronomy. The necessary theorems on the geometry of the sphere (called spherics) were compiled into textbooks, such as the one by Theodosius (3rd or 2nd century BCE) that consolidated the earlier work by Euclid and the work of Autolycus of Pitane (flourished c. 300 BCE) on spherical astronomy. More significant, in the 2nd century BCE the Greeks first came into contact with the fully developed Mesopotamian astronomical systems and took from them many of their observations and parameters (for example, values for the average periods of astronomical phenomena). While retaining their own commitment to geometric models rather than adopting the arithmetic schemes of the Mesopotamians, the Greeks nevertheless followed the Mesopotamians’ lead in seeking a predictive astronomy based on a combination of mathematical theory and observational parameters. They thus made it their goal not merely to describe but to calculate the angular positions of the planets on the basis of the numerical and geometric content of the theory. This major restructuring of Greek astronomy, in both its theoretical and practical respects, was primarily due to Hipparchus (2nd century BCE), whose work was consolidated and further advanced by Ptolemy.

To facilitate their astronomical researches, the Greeks developed techniques for the numerical measurement of angles, a precursor of trigonometry, and produced tables suitable for practical computation. Early efforts to measure the numerical ratios in triangles were made by Archimedes and Aristarchus. Their results were soon extended, and comprehensive treatises on the measurement of chords (in effect, a construction of a table of values equivalent to the trigonometric sine) were produced by Hipparchus and by Menelaus of Alexandria (1st century CE). These works are now lost, but the essential theorems and tables are preserved in Ptolemy’s Almagest (Book I, chapter 10). For computing with angles, the Greeks adopted the Mesopotamian sexagesimal method in arithmetic, whence it survives in the standard units for angles and time employed to this day.

Srinivasa Ramanujan findings:

Srinivasa Ramanujan was one of the world’s greatest mathematicians. His life story, with its humble and sometimes difficult beginnings, is as interesting in its own right as his astonishing work was.

1. The book that started it all
   Srinivasa Ramanujan had his interest in mathematics unlocked by a book. It wasn’t by a famous mathematician, and it wasn’t full of the most up-to-date work, either. The book was A Synopsis of Elementary Results in Pure and Applied Mathematics (1880, revised in 1886), by George Shoobridge Carr. The book consists solely of thousands of theorems, many presented without proofs, and those with proofs only have the briefest. Ramanujan encountered the book in 1903 when he was 15 years old. That the book was not an orderly procession of theorems all tied up with tidy proofs encouraged Ramanujan to jump in and make connections on his own. However, since the proofs included were often just one-liners, Ramanujan had a false impression of the rigor required in mathematics.
2. Early failures
   Despite being a prodigy in mathematics, Ramanujan did not have an auspicious start to his career. He obtained a scholarship to college in 1904, but he quickly lost it by failing in nonmathematical subjects. Another try at college in Madras (now Chennai) also ended poorly when he failed his First Arts exam. It was around this time that he began his famous notebooks. He drifted through poverty until in 1910 when he got an interview with R. Ramachandra Rao, the secretary of the Indian Mathematical Society. Rao was at first doubtful about Ramanujan but eventually recognized his ability and supported him financially.
3. Go west, young man
   Ramanujan rose in prominence among Indian mathematicians, but his colleagues felt that he needed to go to the West to come into contact with the forefront of mathematical research. Ramanujan started writing letters of introduction to professors at the University of Cambridge. His first two letters went unanswered, but his third—of January 16, 1913, to G.H. Hardy—hit its target. Ramanujan included nine pages of mathematics. Some of these results Hardy already knew; others were completely astonishing to him. A correspondence began between the two that culminated in Ramanujan coming to study under Hardy in 1914.
4. Get pi fast
   In his notebooks, Ramanujan wrote down 17 ways to represent 1/pi as an infinite series. Series representations have been known for centuries. For example, the Gregory-Leibniz series, discovered in the 17th century is pi/4 = 1 - ⅓ + ⅕ -1/7 + … However, this series converges extremely slowly; it takes more than 600 terms to settle down at 3.14, let alone the rest of the number. Ramanujan came up with something much more elaborate that got to 1/pi faster: 1/pi = (sqrt(8)/9801) * (1103 + 659832/24591257856 + …). This series gets you to 3.141592 after the first term and adds 8 correct digits per term thereafter. This series was used in 1985 to calculate pi to more than 17 million digits even though it hadn’t yet been proven.
5. Taxicab numbers
   In a famous anecdote, Hardy took a cab to visit Ramanujan. When he got there, he told Ramanujan that the cab’s number, 1729, was “rather a dull one.” Ramanujan said, “No, it is a very interesting number. It is the smallest number expressible as a sum of two cubes in two different ways. That is, 1729 = 1^3 + 12^3 = 9^3 + 10^3. This number is now called the Hardy-Ramanujan number, and the smallest numbers that can be expressed as the sum of two cubes in n different ways have been dubbed taxicab numbers. The next number in the sequence, the smallest number that can be expressed as the sum of two cubes in three different ways, is 87,539,319.
6. 100/100
   Hardy came up with a scale of mathematical ability that went from 0 to 100. He put himself at 25. David Hilbert, the great German mathematician, was at 80. Ramanujan was 100. When he died in 1920 at the age of 32, Ramanujan left behind three notebooks and a sheaf of papers (the “lost notebook”). These notebooks contained thousands of results that are still inspiring mathematical work decades later.