Author: John Quintanilla

I'm a Professor of Mathematics and a University Distinguished Teaching Professor at the University of North Texas. For eight years, I was co-director of Teach North Texas, UNT's program for preparing secondary teachers of mathematics and science.

Solving a Math Competition Problem: Part 1

This series of posts concerns solving the following problem from the 2016 University of Maryland High School Mathematics Competition.

A sphere is divided into regions by 9 planes that are passing through its center. What is the largest possible number of regions that are created on its surface?

a. 2^8

b. 2^9

c. 81

d. 76

e. 74

This series was actually written by my friend Jeff Cagle, department head for mathematics at Chapelgate Christian Academy, as he tried technique after technique to solve this problem. I thought that his resolution to the problem was an excellent example of the process of mathematical problem-solving, and (with his permission) I am posting the process of his solution here. (For the record, I have no doubt that I would not have been able to solve this problem.)

On my first pass, all I could do was to visualize the first three planes, one at the equator, one passing through the prime meridian in Greenwich England, and one passing through the International Date Line. That gave me 2^3=8 regions, so my preliminary conjecture was “b. 2^9”. But I couldn’t prove it. And when I tried to mentally add a fourth plane to my diagram – one starting in Ukraine or something and hitting the equator halfway between the others – I found that I couldn’t clearly see that plane and count the regions formed. That vexed me for a while, and I put it away for the day.

The next day, I realized that I wasn’t going to be able to picture these planes, and I needed to find a way to describe their directions mathematically. The picture I had was of the equatorial plane and a second plane passing through it in the center. That second plane could be rotated any amount around the equator – described by one angle – and then elevated by tilting to a different angle. So I conjectured that two angles uniquely describe each plane: 𝜃 to describe angle around and 𝜙 to describe angle of elevation.

In the shower, I realized that I had just rediscovered latitude and longitude! That made me feel much better about my mathematical description as likely correct.

But now, how to turn the mathematical description into a solution? If I have one plane at (𝜃1,𝜙1), how do I count the regions it creates with the other planes?

Euler and 1,000,009

Here’s a tale of one the great mathematicians of all time that I heard for the first time this year: the great mathematician published a mistake… which, when it occurs today, is highly professionally embarrassing to modern mathematicians. From Mathematics in Ancient Greece:

In a paper published in the year 1774, [Leonhard] Euler listed [1,000,009] as prime. In a subsequent paper Euler corrected his error and gave the prime factors of the integer, adding that one time he had been under the impression that the integer in question admitted of the unique partition

1,000,009 = 1000^2 + 3^2

but that he had since discovered a second partition, namely

1,000,009 = 235^2 + 972^2,

which revealed the composite character of the number.

See Wikipedia and/or Mathworld for the details of how this allowed Euler to factor 1,000,009.

Factoring Mersenne “primes”

I love hearing and telling tales of legendary mathematicians. Today’s tale comes from Frank Nelson Cole and definitely comes from the era before calculators or computers. From Wikipedia:

On October 31, 1903, Cole famously made a presentation to a meeting of the American Mathematical Society where he identified the factors of the Mersenne number 267 − 1, or M67. Édouard Lucas had demonstrated in 1876 that M67 must have factors (i.e., is not prime), but he was unable to determine what those factors were. During Cole’s so-called “lecture”, he approached the chalkboard and in complete silence proceeded to calculate the value of M67, with the result being 147,573,952,589,676,412,927. Cole then moved to the other side of the board and wrote 193,707,721 × 761,838,257,287, and worked through the tedious calculations by hand. Upon completing the multiplication and demonstrating that the result equaled M67, Cole returned to his seat, not having uttered a word during the hour-long presentation. His audience greeted the presentation with a standing ovation. Cole later admitted that finding the factors had taken “three years of Sundays.”

Digital Distraction

From the Chronicle of Higher Education: An Instructor Saw Digital Distraction in Class. So She Showed Students What She’d Seen on Their Screens.

Students get distracted in class, and all the shiny baubles that grab their attention are well chronicled. But what happens when students are presented with the greatest hits from their browsing history for an entire semester?

A graduate-student instructor at the University of Michigan at Ann Arbor, Meg Veitch, did just that. In an effort to keep students focused, she tracked all the times she had spotted them digitally wandering in class. She didn’t have access to their complete browsing history; rather, she used the low-tech method of writing down what she had spotted on students’ screens…

Ms. Veitch, who studies paleontology, presented her findings this week in a PowerPoint show for the class of roughly 160, which gave at least one student a chance to snap and share Ms. Veitch’s observations on Twitter:

A Long-Sought Proof, Found and Almost Lost

I enjoyed this article from Quanta Magazine, both for its mathematical content as well as the human interest story.

A Long-Sought Proof, Found and Almost Lost

From the opening paragraphs:

Known as the Gaussian correlation inequality (GCI), the conjecture originated in the 1950s, was posed in its most elegant form in 1972 and has held mathematicians in its thrall ever since. “I know of people who worked on it for 40 years,” said Donald Richards, a statistician at Pennsylvania State University. “I myself worked on it for 30 years.”

[Thomas] Royen hadn’t given the Gaussian correlation inequality much thought before the “raw idea” for how to prove it came to him over the bathroom sink… In July 2014, still at work on his formulas as a 67-year-old retiree, Royen found that the GCI could be extended into a statement about statistical distributions he had long specialized in. On the morning of the 17th, he saw how to calculate a key derivative for this extended GCI that unlocked the proof. “The evening of this day, my first draft of the proof was written,” he said.

Not knowing LaTeX, the word processer of choice in mathematics, he typed up his calculations in Microsoft Word, and the following month he posted his paper to the academic preprint site arxiv.org. He also sent it to Richards, who had briefly circulated his own failed attempt at a proof of the GCI a year and a half earlier. “I got this article by email from him,” Richards said. “And when I looked at it I knew instantly that it was solved” …

Proofs of obscure provenance are sometimes overlooked at first, but usually not for long: A major paper like Royen’s would normally get submitted and published somewhere like the Annals of Statistics, experts said, and then everybody would hear about it. But Royen, not having a career to advance, chose to skip the slow and often demanding peer-review process typical of top journals. He opted instead for quick publication in the Far East Journal of Theoretical Statistics, a periodical based in Allahabad, India, that was largely unknown to experts and which, on its website, rather suspiciously listed Royen as an editor. (He had agreed to join the editorial board the year before.)

With this red flag emblazoned on it, the proof continued to be ignored… No one is quite sure how, in the 21st century, news of Royen’s proof managed to travel so slowly. “It was clearly a lack of communication in an age where it’s very easy to communicate,” Klartag said.