# The Center of Math Blog

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## 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

### Looking for a weekend read, or a gift for a mathematician in your life?

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.

Read more after the break.

## Tuesday, June 13, 2017

### Problem of the Week: 6-13-17

See how much you can say about the quotient rule for anti-differentiation in this Problem of the Week! Be sure to let us know how you did in the comments or on social media!

Solution below.

## Thursday, June 8, 2017

### Advanced Knowledge Problem of the Week: 6-8-17 [Abstract Algebra]

Check out this #AKPotW, and be sure to let us know what you think in the comments below or on social media!

Solution below the break.

## Tuesday, June 6, 2017

### Problem of the Week: 6-6-17

Let us know what you thought of this problem of the week in the comments below or on social media!

Solution after the break.

## Thursday, June 1, 2017

### Advanced Knowledge Problem of the Week: 6-1-17

Hello! And Welcome to the AKPotW, please let us know how you did in the comments below or on social media :-)

Solution below the break: