A Mathematician for President

"In 1876, a politician made mathematical history. James Abram Garfield, the honorable Congressman from Ohio, published a brand new proof of the Pythagorean Theorem in The New England Journal of Education. He concluded, “We think it something on which the members of both houses can unite without distinction of party.”

You can find the rest of the story, and an interesting ending where I found it, at the blog, Let's Play Math by Denise, who calls herself a home school mom.
The only thing I would add to her well written blog, is that Garfield was not just a politician, he was one of several presidents who came from a mathematical bent. Garfield actually was a professor of mathematics at Hiram College in Ohio for several years before being elected to the Senate in 1859.

A side note about other mathematical Presidents may be of interest to students. Ulysses S Grant wrote that he had hoped to take a position as a math instructor at West Point before the Civil War changed his plans. "Stonewall" Jackson, the Confederate General, was a Professor of Mathematics at VMI (and teachers, if you think your students treat you badly, the VMI cadets dropped rocks from the windows as he walked by trying to bonk him on the head). Washington was an accomplished surveyor, Jefferson's math and architectural talent are well known and Lincoln took time off from his study of law to learn the proofs of all the propositions in the first six books of Euclid so that he could truly understand the meaning of "demonstrate". Eamon de Valera was a leader in the 1916 Easter Rising which proclaimed an Irish republic. Arrested, he was saved from a death sentence because of his American birth and instead received a prison term. He went on to be Prime Minister, and then President of the Republic, but before that, he was a teacher of mathematics. Maybe I should start a list of  "Mathematical Politicians". (Suggestions?)
If you make a copy of Garfield's construction and rotate it 180^o and put the two pieces together you get an image that was presented in a proof from a Chinese block print dating around 40 AD, and reputed by oral tradition to predate the life of Pythagorus in Chinese History.

And why would I know so much about Garfield? His mother's name was Eliza Ballou, which makes her almost family.

Geometry and Understanding

During a conversation with a fellow teacher, he made the statement that he really didn't like geometry, and went on to suggest it might not be a useful course for students. I was somewhat taken aback, and came out with what is probably more an article of faith than a tested fact... "Mathematically, if you really understand it, you can demonstrate it with geometry". Don't be misled; I love the power that comes from algebraic abstraction, but I find that algebraic "proofs" just lack a little something for convincing most students. Something visual seems to have more impact.

The teacher responded by asking (challenging?) for an example, and my first thought was the Pythagorean theorem since it is so ubiquitous in all areas of mathematics. You remember, a² + b² = c²... that one. Now there may be more existing proofs of this theorem than any other in existence. So I drew a right triangle, made three copies and formed them into a square as shown in the figure. Then, with the power of modern educational technology, I moved two of the triangles to get the second figure. Q.E.D. as we say; thus it is shown. Any student who knows the geometric idea of the area of a square can see that the white area in both pictures is the same since the same amount has been removed from it (four congruent triangles). In the first the white area is a square whose sides are formed by the hypotenuses (hypotenii?), c, of the four triangles. In the second it is divided into two smaller squares, one with sides of length a, one with sides of length b. The area then can be expressed as a^2 + b^2 or as c^2, and it is the same area. Somehow I envision a good sixth grader understanding this (and the next sixth grader I see is in for a sit down/talk to so I can find out).







Afterwards it struck me that this was not the best of examples. The Pythagorean theorem is essentially a geometric idea, its about triangles after all. What about an idea from math that has nothing obvious to connect it to geometry . Partitions seemed to fit the bill, and I thought of a simple example that I was sure elementary kids could play with and confirm, and I hope be convinced by a simple geometric argument.
Ok, so for the uninitiated, a partition of a number is just writing it out as a sum of smaller numbers. In order to count them we usually write them from largest addend to smallest. For example 6 = 3 + 2 + 1, so that is one partition of six. There are obviously others, 2+2+2, or 3 + 1 + 1 + 1 would be two more. A common, and not trivial, question is what is the total number of partitions of six? Most elementary students can take an organized attack and come up with the answer. If we include 6 itself, there are eleven of them. 6; 5+1; 4+2; 4+1+1; 3+3; 3+2+1; 3+1+1+1; 2+2+2; 2+ 2+ 1+ 1; 2+1+1+1+1; and of course 1+1+1+1+1+1.
But an interesting thing pops out if we look at some special limitations. There are exactly 3 ways to partition six into three terms; 4+1+1; 3+2+1; 2+2+2. There are also exactly three partitions that use three as the largest number; 3+3; 3+2+1; 3+1+1+1 . Coincidence? Not a chance. The same thing would happen if you partitioned 25 into four terms (or five or six or pick your favorite number). There are exactly the same number of partitions of a number into n differen terms as there are partitions with n as the biggest value. If you have never seen this proof, try to make it clear to yourself why it is true before you read on.
OK, it seems true, but how do we relate that to geometry, and a visual proof? To the rescue comes a 19th Century British Mathematician, Norman M Ferrers, who was a fellow at Gonville and Caius (pronounced Keys) College just down the road here at Cambridge. He seems to have been the first to make a simple discovery about partitions using dots. Here is the Ferrer diagram for two of the unique partitions of six into three groups.







Reading the number of dots in each row we see that 6 = 4 + 1+1; but if we read down the columns we see 3+1+1+1. In the second diagram we see rows of 2+2+2; but columns of 3+3. Can you see that this would apply to any diagram? If there are three rows that add up to some number, then the columns will give us the same total with a three as the largest number. The same thing would be as clear with any other number of rows.

See, Geometry makes it easy to understand, and to explain. Geometry is good. Now go do your homework!