Thursday, January 12, 2017

ПэЧэ, граф Кэли, кубик Рубика и универсалии

Универсального свойства достаточно, чтобы определить объект с точностью до изоморфизма. Таким образом, появляется ещё один способ доказать, что два объекта изоморфны, а именно доказать, что они обладают одинаковым универсальным свойством.

Определение некоего свойства не гарантирует существование объекта, ему удовлетворяющего. Если однако, такой (A, φ) существует, то он единственен. Точнее говоря, он единственен с точностью до единственного изоморфизма.Универсальное_свойство

Свобо́дная гру́ппа в теории групп — группа , для которой существует подмножество  такое, что каждый элемент  записывается единственным образом как произведение конечного числа элементов  и их обратных

Возможно предъявить явную конструкцию свободных групп, доказав тем самым их существование[1][2]Свободная_группа

... любая группа обладает заданием. Задание не обязано быть единственным. Доказать или опровергнуть, что два задания определяют одну и ту же группу сложно (старое название проблемы — одна из проблем Дэна). В общем случае эта проблема алгоритмически неразрешима. ...  Ввиду алгоритмической неразрешимости проблемы, поиск цепочки преобразований Титце одного представления в другое является своего рода искусством.
По заданию группы также сложно определить и другие свойства группы, например её порядок или подгруппу кручения.

Граф Кэли симметрической группы S4

Гру́ппа ку́бика Ру́бика — подгруппа симметрической группы S48

Каждый из поворотов граней кубика Рубика может рассматриваться как элемент симметрической группы множества 48 этикеток кубика Рубика, не являющихся центрами граней. 

Порядок группы  равен[2][3][4][5][6]


Unionpedia is a concept map or semantic network organized like an encyclopedia – dictionary. It gives a brief definition of each concept and its relationships.


Если p — простое число, то любая группа порядка p циклическая и единственна с точностью до изоморфизма


Thursday, January 20, 2011

New theories reveal the nature of numbers

A key creative breakthrough occurred when Emory mathematicians Ken Ono, left, and Zach Kent were hiking. As they walked, they noticed patterns in clumps of trees and began thinking about what it would be like to "walk" amid partition numbers. 

By Carol Clark

For centuries, some of the greatest names in math have tried to make sense of partition numbers, the basis for adding and counting. Many mathematicians added major pieces to the puzzle, but all of them fell short of a full theory to explain partitions. Instead, their work raised more questions about this fundamental area of math.

Now, Emory mathematician Ken Ono is unveiling new theories that answer these famous old questions. (Click here to watch a video of Ono's lecture on the topic.)

Ono and his research team have discovered that partition numbers behave like fractals. They have unlocked the divisibility properties of partitions, and developed a mathematical theory for “seeing” their infinitely repeating superstructure. And they have devised the first finite formula to calculate the partitions of any number.

“Our work brings completely new ideas to the problems,” Ono says. “We prove that partition numbers are ‘fractal’ for every prime. These numbers, in a way we make precise, are self-similar in a shocking way. Our ‘zooming’ procedure resolves several open conjectures, and it will change how mathematicians study partitions.”

The problems of partition numbers "have long fascinated mathematicians," Ono says.

The work was funded by the American Institute of Mathematics (AIM) and the National Science Foundation. Last year, AIM assembled the world’s leading experts on partitions, including Ono, to attack some of the remaining big questions in the field. Ono, who is a chaired professor at both Emory and the University of Wisconsin at Madison, led a team consisting of Jan Bruinier, from the Technical University of Darmstadt in Germany; Amanda Folsom, from Yale; and Zach Kent, a post-doctoral fellow at Emory.

“Ken Ono has achieved absolutely breathtaking breakthroughs in the theory of partitions,” says George Andrews, professor at Pennsylvania State University and president of the American Mathematical Society. “He proved divisibility properties of the basic partition function that are astounding. He went on to provide a superstructure that no one anticipated just a few years ago. He is a phenomenon.”

Child’s play

On the surface, partition numbers seem like mathematical child’s play. A partition of a number is a sequence of positive integers that add up to that number. For example, 4 = 3+1 = 2+2 = 2+1+1 = 1+1+1+1. So we say there are 5 partitions of the number 4.

It sounds simple, and yet the partition numbers grow at an incredible rate. The amount of partitions for the number 10 is 42. For the number 100, the partitions explode to more than 190,000,000.

“Partition numbers are a crazy sequence of integers which race rapidly off to infinity,” Ono says. “This provocative sequence evokes wonder, and has long fascinated mathematicians.”

By definition, partition numbers are tantalizingly simple. But until the breakthroughs by Ono’s team, no one was able to unlock the secret of the complex pattern underlying this rapid growth.

The work of 18th-century mathematician Leonhard Euler (below) led to the first recursive technique forcomputing the partition values of numbers. The method was slow, however, and impractical for large numbers. For the next 150 years, the method was only successfully implemented to compute the first 200 partition numbers.

“In the mathematical universe, that’s like not being able to see further than Mars,” Ono says.

A mathematical telescope

In the early 20th century, Srinivasa Ramanujan and G. H. Hardy invented the circle method, which yielded the first good approximation of the partitions for numbers beyond 200. They essentially gave up on trying to find an exact answer, and settled for an approximation.

“This is like Galileo inventing the telescope, allowing you to see beyond what the naked eye can see, even though the view may be dim,” Ono says.

Ramanujan also noted some strange patterns in partition numbers. In 1919 he wrote: “There appear to be corresponding properties in which the moduli are powers of 5, 7 or 11 … and no simple properties for any moduli involving primes other than these three.”

The legendary Indian mathematician died at the age of 32 before he could explain what he meant by this mysterious quote, now known as Ramanujan’s congruences.

In 1937, Hans Rademacher found an exact formula for calculating partition values. While the method was a big improvement over Euler’s exact formula, it required adding together infinitely many numbers that have infinitely many decimal places. “These numbers are gruesome,” Ono says.

In the ensuing decades, mathematicians have kept building on these breakthroughs, adding more pieces to the puzzle. Despite the advances, they were unable to understand Ramanujan’s enigmatic words, or find a finite formula for the partition numbers.

“We were standing on some huge rocks, where we could see out over this valley and hear the falls, when we realized partition numbers are fractal,” Ono says. Photo by Zach Kent.

Ono’s “dream team” wrestled with the problems for months. “Everything we tried didn’t work,” he says.

A eureka moment happened in September, when Ono and Zach Kent were hiking to Tallulah Falls in northern Georgia. As they walked through the woods, noticing patterns in clumps of trees, Ono and Kent began thinking about what it would be like to “walk” amid partition numbers.

“We were standing on some huge rocks, where we could see out over this valley and hear the falls, when we realized partition numbers are fractal,” Ono says. “We both just started laughing.”

The term fractal was invented in 1980 by Benoit Mandelbrot, to describe what seem like irregularities in the geometry of natural forms. The more a viewer zooms into “rough” natural forms, the clearer it becomes that they actually consist of repeating patterns (see youtube video, below). Not only are fractals beautiful, they have immense practical value in fields as diverse as art to medicine.