# The Power of Partitions

Writing a whole number as the sum of smaller numbers springs a mathematical surprise

Just a year before his death in 1920 at the age of 32, mathematician Srinivasa Ramanujan came upon a remarkable pattern in a special list of whole numbers.

The list represented counts of how many ways a given whole number can be expressed as a sum of positive integers. For example, 4 can be written as 3 + 1, 2 + 2, 2 + 1 + 1, and 1 + 1 + 1 + 1. Including 4 itself but excluding different arrangements of the same integers (2 + 1 + 1 is considered the same as 1 + 2 + 1), there are five distinct possibilities, or so-called partitions, of the number 4. Similarly, the integer 5 has seven partitions.

The list that Ramanujan perused gave for each of the first 200 integers, the number of their partitions, which range from 1 to 3,972,999,029,388.

Ramanujan noticed that, starting with 4, the number of partitions for every fifth integer is a multiple of 5. For instance, the number of partitions for 9 is 30 and for 14 is 135.

He discovered several more such patterns. Starting with 5, the number of partitions for every seventh integer is a multiple of 7, and starting with 6, the number of partitions for every 11th integer is a multiple of 11. Moreover, similar relationships occur where the interval between the chosen integers is a power of 5, 7, or 11 or a product of these powers. Ramanujan went on to prove rigorously that these patterns hold not only for the 200 partition numbers in his table but also for all higher numbers.

It was a curious discovery. Nothing in the definition of partitions hinted that such relationships, called congruences, should exist or that the prime numbers 5, 7, and 11 should play a special role.

After many decades of nearly fruitless searching that yielded just one or two apparently isolated examples of large numbers that fit the pattern, mathematicians came to believe that no other congruences exist. Those found by Ramanujan and the later mathematicians were thought to be little more than numerical flukes.

Indeed, “it was really believed that there would probably never be any new major discoveries regarding partition congruences,” says George E. Andrews of Pennsylvania State University in State College.

In a startling turnabout, mathematician Ken Ono has now proved that infinitely many such relationships occur. “This was really quite unexpected,” says number theorist Ono, who holds positions at both Penn State and the University of Wisconsin-Madison. He described his results in the January Annals of Mathematics.

“Ono’s work is really spectacular,” comments Bruce C. Berndt of the University of Illinois at Urbana-Champaign. “This certainly must rank as the most important work on partition congruences since the epic work of Ramanujan.”

“It’s an extraordinary discovery,” agrees Andrew M. Granville of the University of Georgia in Athens.

Moreover, Ono’s partition research has intriguing, unexpected links to complex mathematical ideas and methods that earlier led to proofs of Fermat’s last theorem (SN: 11/5/94, p. 295) and the Taniyama-Shimura conjecture (SN: 10/2/99, p. 221). It has opened up new avenues for studying some of the most important, but difficult, questions in number theory, Granville says.

Hooked on math

Born into poverty, Ramanujan grew up in southern India, and although he had little formal training in mathematics, he became hooked on mathematics. He spent the years between 1903 and 1913 cramming notebooks with page after page of mathematical formulas and relationships that he had uncovered (SN: 4/25/87, p. 266).

Ramanujan’s life as a professional mathematician began in 1914 when he accepted an invitation from the prominent British mathematician G.H. Hardy to come to Cambridge University. He spent 5 years in England, publishing many papers and achieving international recognition for his mathematical research.

Though his work was cut short by a mysterious illness that brought him back to India for the final year of his life, Ramanujan’s work has remained a subject of considerable interest. For the past 2 decades, Berndt has been going through Ramanujan’s scribbled notes, systematically deciphering, writing out, and proving each of the numerous theorems Ramanujan had formulated.

 Partitions of 5 The integer 5 has seven partitions: 5 4 + 1 3 + 2 3 + 1 + 1 2 + 2 + 1 2 + 1 + 1 + 1 1 + 1 + 1 + 1 + 1

Two years ago, Berndt was examining an unpublished, handwritten Ramanujan manuscript on partitions. “It contained claims that had never been proved,” Berndt remarks. So he recruited Ono, an expert on partitions from a modern perspective, to help him fill in the details and provide the necessary proofs.

“I had never read any of Ramanujan’s notebooks, though I was familiar with a lot of what he had done through the writings of more modern mathematicians,” Ono says. “I didn’t suspect that I would learn anything from studying Ramanujan’s notes.”

In fact, the manuscript didn’t contain anything startling or new. However, Ramanujan had written out one mathematical expression, or identity, in a manner that to Ono seemed particularly awkward.

“The first time I saw it, I wrote to Berndt saying this can’t be right,” Ono recalls. Nonetheless, after Ono translated the expression into modern mathematical language, it made perfect sense.

Yet the apparent awkwardness of Ramanujan’s original formulation bothered Ono. It got him to think deeply about the many different ways in which mathematicians can express identities. In the course of that rethinking, he obtained crucial insights that led him to tackle the question of partition congruences—something that he would not otherwise have attempted.

“I learned a valuable lesson,” Ono remarks. “It sometimes really pays to read the original.”

Tour de force

In a remarkable tour de force, Ono proved that partition congruences can be found not only for the prime numbers 5, 7, and 11 but also for all larger primes. To do so, he had to exploit a previously unsuspected link between partition numbers and complex mathematical objects called modular forms.

“Modular forms lie at the heart of modern number theory,” says Scott Ahlgren of Colgate University in Hamilton, N.Y.

They are special types of mathematical relationships that involve so-called complex numbers, which have a real and an imaginary component. In effect, the relationships represent particular types of repeating patterns, roughly analogous to the way a trigonometric function such as sine represents a repeating pattern for ordinary numbers.

Mathematicians have identified many different types of modular forms. The connection between certain modular forms and elliptic curves helped Andrew Wiles of Princeton University prove Fermat’s last theorem in 1993.

Applying modular-form theory, as developed by other mathematicians, Ono uncovered a connection between partition numbers and specific modular forms. He used that relationship to establish, in effect, the existence of congruences involving prime-number divisors among the partition numbers.

Unlike many previous advances in partition theory, Ono’s research involved no computation and relied heavily on theory. “What I find particularly appealing about this approach is that it uses the most powerful tools in modern number theory to attack a classical problem,” Ahlgren says.

Interestingly, although Ono proved the existence of these congruences, he provided only one explicit example. In this new congruence, the starting number is 111,247, with each successive step to the next integer going up by 594 x 13. The corresponding partitions are then multiples of the prime number 13.

Ono, however, didn’t have an effective recipe, or algorithm, for generating examples. But Rhiannon L. Weaver, an undergraduate at Penn State, found a simple criterion for identifying the start of a progression. She then developed an algorithm, working with primes from 13 to 31, to obtain more than 70,000 new congruences.

 Integer Numberof Partitions 1 1 2 2 3 3 4 5 5 7 6 11 7 15 8 22 9 30 10 42 11 56 12 77 13 101 14 135 15 176 16 231 17 297 18 385 19 490 20 627 21 792 22 1,002 23 1,255 24 1,575 25 1,958 26 2,436 27 3,010 28 3,718 29 4,565 30 5,604 31 6,842 32 8,349 33 10,143 34 12,310 35 14,883 36 17,977 37 21,637 38 26,015 39 31,185 40 37,338 41 44,583 42 53,174 43 63,261 44 75,175 45 89,134 Thenumber of partitions for each integer from 1 to 45. The box colored green represents a partition number that’s a multiple of both 5 and 7, and the orange box indicates a multiple of 5 and 11. Yellow boxes indicate those numbers that are multiples of only 5; blue represents multiples of only 7; and red indicates multiples of only 11.

“This wasn’t a trivial exercise,” Ono says. “It was a great piece of work.” Applying Weaver’s method, a researcher can now readily write a computer program to find thousands of additional examples, he explains.

The newfound congruences also show why mathematicians had failed to come up with many additional examples by trial and error. “The numbers involved are very big,” Granville remarks. Moreover, “even now that we know where to look, I don’t think we would have spotted them from raw computation.”

In another recent development, Ahlgren has extended Ono’s results to establish the existence of congruences that work with multiples of composite numbers, which consist of primes multiplied together, as long as the numbers aren’t divisible by 2 or 3.

Ahlgren reported his results in May at the Millennial Conference on Number Theory held at Illinois.

“It is now apparent that Ramanujan-type congruences are plentiful,” Ono says. “However, it is typical that such congruences are monstrous.”

New role

Ono’s results have already sparked a considerable amount of research activity. “It’s amazing how much more we know now than we did just last year,” Ahlgren says.

Despite these advances, however, mathematicians still don’t know whether there are congruences that involve multiples of 2 or 3. Ono’s methods don’t work for these particular cases, and researchers must develop new tools to hunt for such relationships.

At the same time, Ono has given partition numbers an exciting, new role in mathematics. “Partitions are much more than just counts of how to add up numbers,” Ono says. “They are a vehicle for testing some of the most important conjectures about [mathematical] objects that we can barely get a handle on otherwise.”

Indeed, studying partitions could lead to new insights into the theory of modular forms and illuminate its connections with important, unsolved questions in number theory, such as the so-called Swinnerton-Dyer conjecture. The Clay Mathematics Institute in Cambridge, Mass., recently listed this problem as one of the top seven questions in mathematics and offered a \$1 million prize for its solution. The conjecture concerns a criterion for deciding whether certain equations have whole-number solutions.

The study of partitions has long been one of the mainstays of number theory and rivals the study of primes for intrinsic mathematical appeal, Andrews says.

“Primes form the basis for multiplication, and the study of partitions is grounded in the addition of integers,” he notes. “What is intriguing is the fact that such an elementary idea can have theorems as subtle as those of Ono.”