# The Center of Math Blog

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## Thursday, October 19, 2017

### Think Thursday 10-19-17: Chessboard and Dominoes

Check out this logic based Think Thursday Problem!
Be sure to let us know how you solved it in the comments below or on social media!

Solution below.

## Tuesday, October 17, 2017

### Problem of the Week 10-17-17: Positive Integer Triple

Check out this Problem of the Week.
Be sure to let us know how you solved it in the comments below or on social media!

Solution below.

## Thursday, October 12, 2017

### Think Thursday 10-12-17: Rays through Squares

Check out this logic based Think Thursday Problem!
Be sure to let us know how you solved it in the comments below or on social media!

Solution below.

## Tuesday, October 10, 2017

### Problem of the Week 10-10-17: Marking the Grid [Configurations]

Check out this Problem of the Week.
Be sure to let us know how you solved it in the comments below or on social media!

Solution below.

## Thursday, October 5, 2017

### Think Thursday 10-5-17: Cheryl's Birthday [Deductive Reasoning]

Check out this logic based Think Thursday Problem!
Be sure to let us know how you solved it in the comments below or on social media!

Solution below.

## Tuesday, October 3, 2017

### Problem of the Week 10-3-17: Folding a Piece of Paper [Geometry]

Check out this Problem of the Week.
Be sure to let us know how you solved it in the comments below or on social media!

Solution below.

## Thursday, September 28, 2017

### Think Thursday 9-27-17: Bridge Crossing [Logic]

Check out this logic based Think Thursday Problem!
Be sure to let us know how you solved it in the comments below or on social media!
This problem was originally posted by MAA online.

Solution below.

## Tuesday, September 26, 2017

### Problem of the Week 9-26-17: Choosing balls from Urns [Probability]

Check out this Problem of the Week.
Be sure to let us know how you solved it in the comments below or on social media!

Solution below.

## Thursday, September 21, 2017

### Think Thursday 9-21-17: Twelve Coins

Check out this logic based Think Thursday Problem!
Be sure to let us know how you solved it in the comments below or on social media!

Solution below.

## Tuesday, September 19, 2017

### Problem of the Week 9-19-17: Four Digit Numbers

Check out this Problem of the Week.
Be sure to let us know how you solved it in the comments below or on social media!

Solution below.

## Thursday, September 14, 2017

### Think Thursday 9-14-17: Tiling with Dominoes

Welcome to our first Think Thursday Problem!
This series aims to introduce logic based problems, puzzles, and other tricky brain teasers. The problems featured here are Math related, but do not require a extensive knowledge of Mathematics to solve. We hope you enjoy this new series!

Be sure to let us know how you solved it in the comments below or on social media!

Solution below.

## Tuesday, September 12, 2017

### Problem of the Week 9/12/1: Variables [Algebra]

Check out this Problem of the Week and enjoy this math joke.

Why did the variable break up with the constant?
Because the constant was incapable of change.

Be sure to let us know how you solved it in the comments below or on social media!

Solution below.

## Friday, September 8, 2017

### Episode 10: Prime Number Theorem [#MathChops]

This episode of #MathChops focuses on prime numbers and their theorem. The prime number theorem describes the distribution of prime numbers becomes much more sparse and numbers get bigger. This theorem helps us quantify how many prime numbers there are less than a specific number n. The proof itself for this theorem is quite extensive, but it is still fascinating to learn about. Watch our video below to learn more about the prime number theorem.

## Tuesday, September 5, 2017

### Problem of the Week 9-5-17: Find P(0) [Algebra]

Check out this Problem of the Week.
Be sure to let us know how you solved it in the comments below or on social media!

Solution below.

## Thursday, August 31, 2017

### Episode 9: The Limit Definition of e [#MathChops]

The history of ‘e’ is a tangled one, one which would warrant an entire dedicated book to parse through mathematics to the original conception of the transcendental number. Even before e’s enigmatic beauty was fully unearthed, people using mathematics to solve real world problems encountered the number many times, and understood it enough to work it into their solutions. A good example of this is when e shows up in compound interest. Bankers found out that as the number of times one took annual compound interest grew to infinity, the rate of growth approached e! Watch the video to see two mathematical proofs of our statement, using two definitions of e.

### Advanced Knowledge Problem of the Week: Drunkard's Walk [Probability]

Be sure to let us know how you solved this in the comments below or on social media!

Solution below.

## Tuesday, August 29, 2017

### Problem of the Week 8-29-17: Pool Table

Check out this Problem of the Week.
Be sure to let us know how you solved it in the comments below or on social media!

Solution below.

## Thursday, August 24, 2017

### Advanced Knowledge Problem of the Week: Finding a Perfect Square Solution [Algebra]

Be sure to let us know how you solved this in the comments below or on social media!

Solution below.

## Wednesday, August 23, 2017

### Back to School Math Courses Review Guide

Oh no, back to school is right around the corner!

We know it can be hard to jump straight back into Math classes during the first few days after a summer away. Lucky for you we have arranged some of of our Youtube channel videos into a helpful guide to make sure you are on your game in the first week of class. Check them out below!

## Tuesday, August 22, 2017

### Problem of the Week 8-21-17: Seven Pointed Star [Geometry]

Check out this Algebraic Problem of the Week.
Be sure to let us know how you solved it in the comments below or on social media!

Solution below.

## Friday, August 18, 2017

### Episode 8: Infinite Primes [#MathChops]

Back in 300 BC, Euclid proved that there were an infinite number of primes. He used line segments to show that some line lengths could only be made up from single-unit line lengths and not lines with lengths of 2, 3, etc. These line lengths represented prime numbers. This proof has the same principle but is a little different than Euclid's and uses proof by contradiction. Take a look at this simple proof which shows that primes are infinite!

## Thursday, August 17, 2017

### Advanced Knowledge Problem of the Week: Infinite Product of a Sequence [Convergence]

Be sure to let us know how you solved this in the comments below or on social media!

Solution below.

## Tuesday, August 15, 2017

### Problem of the Week 8-15-17 [Algebra]

Check out this Algebraic Problem of the Week.
Be sure to let us know how you solved it in the comments below or on social media!

Solution below.

## Thursday, August 10, 2017

### Advanced Knowledge Problem of the Week: Website Traffic as a Poisson Process [Stochastic Processes]

Be sure to let us know how you solved this in the comments below or on social media!

Solution below.

## Tuesday, August 8, 2017

### Problem of the Week 8-8-17 [Distance]

Check out this Problem of the Week.
Be sure to let us know how you solved it in the comments below or on social media!

Solution below.

## Friday, August 4, 2017

### Episode 7: Gauss and Triangular Numbers [#MathChops]

The mythology behind this fairly simple proof is what makes it one of the most popular proofs in math classes across the world. The story follows a young Carl Friedrich Gauss, whose first grade teacher asked the class to add up the numbers 1 to 100 in order to pass a good amount of time. Before the teacher had time to start grading papers, Gauss handed in his assignment. Watch the video to find out Gauss’ observation that is now one of the most famous math proofs out there.

## Thursday, July 27, 2017

### Advanced Knowledge Problem of the Week: Is the vector in the span? [Linear Algebra]

Be sure to let us know how you solved this in the comments below or on social media!

Solution below.

## Tuesday, July 25, 2017

### Problem of the Week 7/25/17 Math inspired Alphametic [puzzle]

Check out this Mathematics inspired Alphametic Problem of the Week
Be sure to let us know how you solved it in the comments below or on social media!

The two basic rules for solving alphametics are as follows:
Each letter must be represented by a different digit. If the letter is used more than once, it must be represented by the same digit.
Once you substitute digits for all your letters, you must end up with an accurate addition problem.

Solution below.

## Friday, July 21, 2017

### Episode 6: Area of a Circle [#MathChops]

This problem of determining the area of a circle, or better defined as the area inside of a circle, was a huge dilemma in the field of mathematics. It was not until the mid 200's BC when Archimedes began to anticipate modern calculus and analysis though concepts of infinitesimals and exhaustion, which he used to solve this major challenge of finding the area of a circle.

Archimedes' method of finding the area is described as "squaring the circle", which is trying to find the square that has the same enclosed area as a circle of a given radius. Using this and also using a method where he approximated the area of a circle with other, known shapes such as squares and hexagons, Archimedes was able to determine the area inside of a circle. Take a look at the proof to see how Archimedes came up with the formula we know today:

## Thursday, July 20, 2017

### Advanced Knowledge Problem of the Week: Game of Thrones Committee [Combinatorics]

Be sure to let us know how you solved this in the comments below or on social media!

Solution below.

## Tuesday, July 18, 2017

### Problem of the week 7/18/17 [geometry]

#### How do you draw a circle? We analyzed 100,000 drawings to show how culture shapes our instincts

Be sure to let us know how you solved it in the comments below or on social media!

Solution below.

## Friday, July 14, 2017

### Episode 5: Königsberg Bridge Problem (Seven Bridges) [#MathChops]

The advent of graph theory, from the mind of Leonhard Euler, came from a long-standing problem for the people of Königsberg. The problem was that no couple had a long and happy marriage, if they were married in Königsberg. As tradition dictated, a newlywed couple had one chance to travel across Königsberg’s four land masses using each of the seven bridges once and only once. If the two lovers could complete this seemingly simple task, their marriage would be long and happy. Years went by and nobody could complete to task, until Euler constructed a mathematical object that broke the curse of Königsberg… a graph!

Watch the proof proposed by Euler below to learn how mathematical abstraction created a whole new field of math, which is now regarded as an important predecessor to topology. Euler’s invention itself is remarkable, but the implications to mathematical philosophy reveals something very deep in the heart of mathematics. Namely, the art of abstraction to gain a better understanding of certain truths inherent in life’s situations.

The Königsberg Bridge Problem, and its solving:

## Thursday, July 13, 2017

### Advanced Knowledge Problem of the Week: 7-13-17 [Analysis]

Be sure to let us know how you prove this in the comments below or on social media!

Solution below.

## Tuesday, July 11, 2017

### Problem of the Week: 7-11-17 [Calculus I]

Be sure to let us know how you solved it in the comments below or on social media!

Solution below.

## Thursday, July 6, 2017

### Episode 4: Uncountability of Real Numbers [#MathChops]

This week’s Top Pop Math Chop comes from Georg Cantor, who first solved this piece of set theory in 1891. He presented this as a mathematical proof which showed it was impossible to link infinite sets with an infinite set of the natural numbers. This is known today as Cantor’s diagonal argument, which he proved using binary numbers.

Cantor showed that if he has a list of binary numbers, takes one digit from each going diagonally, produces a new number, and swaps every single digit with a corresponding 1 or 0 (if is a 1 it becomes 0 and vice versa), that the number will be different than every other binary number listed before it. This is because in the first number the first digit is different, so it’s definitely different than the new number; in the second number the second digit is different than the second digit in the new number and so on.

You can do this same thing with real numbers, and produce infinite decimals between 0 and 1. This shows the real numbers are uncountable.

Check out the video below explaining Georg Cantor’s proof:

### Advanced Knowledge Problem of the Week: 7-6-17 [calculus]

Be sure to let us know how you did in the comments below or on social media!

This week's problem comes from our textbook, Worldwide Multivariable Calculus, so feel free to check it out or any other affordable texts we offer. Enjoy this problem and try to find its relation to July 4th!

Solution below.

## Friday, June 30, 2017

### Problem of the Week: 7-4-17 [Geometry]

Be sure to let us know how you did in the comments below or on social media!

Solution below.

### Episode 3: All Horses Are the Same Color -- Equine Monochromaticity [#MathChops]

Obviously, this theorem is false, but it is a good way to show off your math chops and confuse a friend who may be taking an introductory course in math reasoning. This ‘proof’ is purely for fun, but does point out an important part of inductive proofs, which is that the assumption for the ‘n’th case must imply our statement is true in the ‘n+1’th case for any arbitrary n. Take what you will from this proof, but it reminds me of a joke I heard once.

A mathematician, physicist, and engineer are on a train in spain and see a white horse. The engineer remarks, “all horses are white!” to which the physicist and mathematician shake their heads. “No no no,” says the physicist, “what this means is that some horses in spain are white.” to which the mathematician shakes his head. The mathematician thinks for a little, and says “In passing we saw a white horse grazing in the plains of spain; therefore, there exists at least one horse in spain, of which at least one side is white.” and the three go about their day.

Proof

## Thursday, June 29, 2017

### Advanced Knowledge Problem of the Week: 6-29-17

Be sure to let us know how you did in the comments below or on social media!

Solution below.

## Tuesday, June 27, 2017

### Problem of the Week: 6-27-17 [Linear Algebra]

Check out this #PotW about properties of orthogonal matrices! as always, let us know what you think about it in the comments below or on social media!

Solution below the break.

## Friday, June 23, 2017

### #MathChops Episode 2: Proof That the Irrationals Are a Dense Set Within the Reals

The first conception of this episode was to prove that the rationals are a dense within the reals, which is an algebraic proof showing that between any two real numbers, there is a rational number. This proof does not define the real numbers, and treats them as some empirical fact that you know; yet, once the real numbers are constructed, the proof is really trivial. The proof used in this episode utilizes an analytic definition of dense sets: if a set A’ along with its limit points equals the B’, then A’ is a dense set within B’. You will see that we construct the reals in such a way that the rationals are dense within the reals. But first, a little background.

First, we construct the natural numbers using Peano’s Axioms, and the integers can be constructed many different ways from the natural numbers (think including additive inverses). From the integers, the rational numbers are all ratios of two integers. These ratios can be thought of as finite decimal expansions, and we will construct the real numbers using Dedekind cuts. To define a real number, we chop the number line at the end of an infinite decimal expansion, and call the set of all rational numbers less than that cut the real number. Now of course, this is defining any real number as the limit point of a rational sequence, making the closure of the rationals (the rationals along with their limit points) the reals. The proof that the irrationals are a dense set within the reals is less obvious.

 Construction of the rationals, from AMS.
We need to find an irrational sequence that converges to a rational number (let’s choose 1,  and get any rational number by multiplying our sequence). After some thought, the sequence
a_n = 1 + \frac{\sqrt{2}}{n} is a sequence of irrational terms whose limit point is a rational number. Thus, the irrationals are a dense set within the reals.

Proof

## Thursday, June 22, 2017

### Advanced Knowledge Problem of the Week: 6-22-17 [dynamics]

Check out this problem on dynamical systems! Let us know how you did in the comments below or on social media!

Solution below.

## Tuesday, June 20, 2017

### Problem of the Week: 6-19-17 [Calculus]

Check out this week's problem of the week, finding the optimum way to craft a boxes net. Let us know how you did in the comments below or on social media!

Solution below the break.

## Friday, June 16, 2017

### New Publication: Some of Infinity by David Craft

Consider the latest publication from the Worldwide Center of Mathematics, Some of Infinity by David Craft. The book takes its time, meandering from topic to topic, numbers, infinity, fractals, calculus, topology, and takes the reader through these subjects from conception to completion. This way of going through each section makes for a good change of pace for anyone who reads a lot of math books, which can zoom through interesting points; and can be a thoughtful introduction to math material for anyone who doesn't.

Buy the digital or print version here.

Watch our review of the book:

### #MathChops Episode 1: Proof of the Pythagorean Theorem

One of the cornerstones in Mathematics was proven by Pythagoras around 520 BC. Today we know this as the Pythagorean theorem, which states the sum of the squares of two sides of a triangle equal the square of its hypotenuse (a2 + b2 = c2). Pythagoras not only discovered this theorem, but he also started a philosophical and religious school where his followers worked and lived. They were known as the Pythagoreans and they lived by a specific set of rules, which dictated when they spoke, what they wore, and what they ate. Their lives were dedicated to universal discoveries and proving theorems. Pythagoras was the Master of these men and women, who were known as mathematikoi.
 A graphic from Some of Infinity.

In our book, Some of Infinity, the author, David Craft, briefly talks about the Pythagoreans and goes on to prove the Pythagorean theorem. He touches on numerous sections of Mathematics such as Numbers, Infinity, Probability, Fractals, Calculus, and more. The idea for the book came about from trying to convince his friends that math is fun and cool. He does a very good job of portraying that math actually is fun and interesting, while keeping the reader engaged with cool puzzles and riddles.

Watch the proof here!

### New Series: Top Pop Math Chops

Top Pop Math Chops is the Worldwide Center of Mathematic's new series that will go into some popular, and often times important, proofs across many facets of mathematics; from simple geometry, to calculus and beyond. Some proofs you will recognize because you use the result in day-to-day mathematics, and we think it is important that the actual mathematics behind the proof is laid out clearly. The scope of this series is wide, ranging from ancient techniques to prove mathematical truths, to modern methods and intuitions.

We hope you enjoy our journey through the fun, important, and interesting proofs that every math enthusiast should know! keep in touch with @centerofmath on Facebook, Twitter, or G+ using #MathChops to let us know what you think, or if there are any proofs you think we should cover.

Watch the introduction episode now!

## Thursday, June 15, 2017

### Advanced knowledge Problem of the Week: 6-14-17 [Real Analysis]

Check out this exercise covering uniform and pointwise convergence of sequences of functions! Let us know how you did in the comments below or on social media!

Solution below the break.

## Wednesday, June 14, 2017

### Flag Day and Mathematics

Flag Day celebrates the adoption of the United States' flag on this day in 1777. The symbol for unity, spread over thirteen stripes and fifty stars, stands tall as a momentous proclamation of the United States' values. Over the years, with the growth of our country, the flag of the U.S. has also changed, from thirteen stars to fifty. With each revision of the flag's design, a great deal of thought goes into the arrangement of our star spangled banner; and while mathematics is not always considered in this process, we know math is capable of bringing to our attention beauty, so today we will consider how math could play into our flag.