Katz concentrates almost entirely on Euclid, and with Euclid he concentrates on two of the biggest theorems, the Pythagorean theorem and the construction of the pentagon. What I found most interesting was an implied criticism of the traditional way of writing mathematics. He writes (pp. 36, 38, 49):
"Euclid's Elements ... to the modern reader is incredibly dull. There are no examples; there is no motivation; there are no witty remarks; there is no calculation. There are simply definitions, axioms, theorems, and proofs.... If one reads Book I from the beginning, one never has any idea what will come next. It is only when one gets to the end of the book, where Euclid proves the Pythagorean theorm, that one realizes that Book I's basic purpose is to lead to the proof of that result.... As usual, Euclid does not show how he arrived at the [pentagon] construction ..."
It seems like Prof. Katz has been reading one of my favorite books, "Proofs and Refutations: The Logic of Mathematical Discovery", by I. Lakatos (Cambridge UP, 1976). (That would be a good choice for a book review, come to think of it.) Lakatos sees Euclid as the source of the Original Sin of mathematicians, our careful and proud presentation of ideas in the most logical order, which leaves our students spending most of their time asking, "Why are we doing this?"
By skipping from Pythagoras to Euclid, Katz misses out the Greek alphabetic system of numerals, such major mathematicians as Anaxagoras and Eudoxus, and two quirky, thought-provoking figures, Zeno and Hippias (of whom more below).
Here are some things that were unclear to me [after reading Allen in 2003] until I read up on them in Heath's book.
Having constructed square numbers and triangular numbers (1,3,6,10,...) as expanding arrays of dots, the Pythagoreans or their successors tried to do the same with pentagons, etc. This does not work out quite so neatly, since the dots can't form an equally spaced, regular lattice. Nevertheless, the generalization of the triangle and square constructions is this (stated for the pentagon, for definiteness): Start with one point and add 4 more points to make a pentagon. Now leave two adjacent sides of the pentagon alone, but build out the pattern in the other three directions -- thus adding successive gnomons that consist of 3 straight segments. At each step the number of dots on each segment increases by 1, so that the total number of dots in the new gnomon is larger by 3 than the old gnomon. (This generalizes the fact for the square that the size of each half of the gnomon (not counting the point at the corner) increases by 1 at each step.)
More generally, for n-gonal numbers the successive gnomons differ by n-2. Thus the total number of dots in a polygonal number is the sum of a finite arithmetic progression, and in this way the Greeks were led to some of the summation formulas that we now encounter in calculus books as examples of functions whose Riemann sums can be evaluated exactly, or in other textbooks as examples of proofs by mathematical induction.
The diagrams of pentagonal and hexagonal numbers in Allen (p. 16) and Heath may look like 3-dimensional objects (pyramids or Christmas trees), but they are not intended as such. Later Greeks (principal author Nichomachus, c. 100 AD) did, however, associate numbers with 3-dimensional shapes, notably cubes (of course) and pyramids (take a point, add a triangle of 3 points, then a triangle of 6 points, etc.; similarly for the sequence of n-gonal numbers for any n). This game further extended the collection of finite series whose sums were known.
Here is a more modern analogy: A slide rule allows you to calculate (or "look up") the logarithm or square root of any number. But you can't construct an accurate slide rule without being able to calculate those functions. (If you are too young to know what a slide rule is, we'll get back to it in the Renaissance.)
How the curve is used to trisect angles and to square the circle is described by Heath on pages 227-229. Rendering that discussion in HTML is beyond my powers, so we now branch off to a TeX/PDF continuation.