Denver airport, Weierstrass, and A&S

Last night I was driving toward the Denver airport and the airport reminded me of the cover of Abramowitz and Stegun’s Handbook of Mathematical Functions.

Here’s the airport:

Denver airport

And here’s the book cover:

I’ve written about the image on book cover before. Someone asked me what function it graphed and I decided it was probably the Weierstrass ℘ function.

For more on Weierstrass’ elliptic function and why I think that’s what’s on the cover of A&S, see this post.

Photo of Denver airport via Wikipedia.

Flying through a 3D fractal

A Menger sponge is created by starting with a cube a recursively removing chunks of it. Draw a 3×3 grid on one face of the cube, then remove the middle square, all the way through the cube. Then do the same for each of the eight remaining squares. Repeat this process over and over, and do it for each face.

The holes are all rectangular, so it’s surprising that the geometry is so varied when you slice open a Menger sponge. For example, when you cut it on the diagonal, you can see stars! (I wrote about this here.)

I mentioned this blog post to a friend at Go 3D Now, a company that does 3D scanning and animation, and he created the video below. The video starts out by taking you through the sponge, then at about the half way part the sponge splits apart.

Computing harmonic numbers

The harmonic numbers are defined by

Harmonic numbers are sort of a discrete analog of logarithms since

\log n = \int_1^n \frac{1}{x} \, dx

As n goes to infinity, the difference between Hn and log n is Euler’s constant γ = 0.57721… [1]

How would you compute Hn? For small n, simply use the definition. But if n is very large, there’s a way to approximate Hn without having to do a large sum.

Since in the limit Hn – log n goes to γ, a crude approximation would be

H_n \approx \log n + \gamma

But we could do much better by adding a couple terms to the approximation above. [2] That is,

H_n \approx \log n + \gamma + \frac{1}{2n} - \frac{1}{12n^2}

The error in the approximation above is between 0 and 1/120n4.

So if you used this to compute the 1000th harmonic number, the error would be less than one part in 120,000,000,000,000. Said another way, for n = 1000 the approximation differs from the exact value in the 15th significant digit, approximately the resolution of floating point numbers (i.e. IEEE 754 double precision).

And the formula is even more accurate for larger n. If we wanted to compute the millionth harmonic number, the error in our approximation would be somewhere around the 26th decimal place.

* * *

[1] See Julian Havil’s excellent Gamma: Exploring Euler’s Constant. It’s popular-level book, but more sophisticated than most such books.

[2] There’s a sequence of increasingly accurate approximations that keep adding reciprocals of even powers of n, based on truncating an asymptotic series. See Concrete Mathematics for details.

Quantile-quantile plots and powers of 3/2

This post serves two purposes. It will empirically explore a question in number theory and demonstrate quantile-quantile (q-q) plots. It will shed light on a question raised in the previous post. And if you’re not familiar with q-q plots, it will serve as an introduction to such plots.

The previous post said that for almost all x > 1, the fractional parts of the powers of x are uniformly distributed. Although this is true for almost all x, it can be hard to establish for any particular x. The previous post ended with the question of whether the fractional parts of the powers of 3/2 are uniformly distributed.

First, lets just plot the sequence (3/2)n mod 1.

powers of 3/2 mod 1

Looks kinda random. But is it uniformly distributed? One way to tell would be to look at the empirical cumulative distribution function (ECDF) and see how it compares to a uniform cumulative distribution function. This is what a quantile-quantile plot does. In our case we’re looking to see whether something has a uniform distribution, but you could use a q-q plot for any distribution. It may be most often used to test normality by looking at whether the ECDF looks like a normal CDF.

If a sequence is uniformly distributed, we would expect 10% of the values to be less than 0.1. We would expect 20% of the values to be less than 0.2. Etc. In other words, we’d expect the quantiles to line up with their theoretical values, hence the name “quantile-quantile” plot. On the horizontal axis we plot uniform values between 0 and 1. On the vertical axis we plot the sorted values of (3/2)n mod 1.

qq plot of powers of 3/2 mod 1

A qq-plot indicates a good fit when values line up near the diagonal, as they do here.

For contrast, let’s look at a qq-plot for the powers of the plastic constant mod 1.

qq plot of powers of the plastic constant

Here we get something very far from the diagonal line. The plot is flat on the left because many of the values are near 0, and it’s flat on the right because many values are near 1.

Incidentally, the Kolmogorov-Smirnov goodness of fit test is basically an attempt to quantify the impression you get from looking at a q-q plot. It’s based on a statistic that measures how far apart the empirical CDF and theoretical CDF are.

Uniform distribution of powers mod 1

A few days ago I wrote about how powers of the golden ratio are nearly integers but powers of π are not. This post is similar but takes a little different perspective. Instead of looking at how close powers are to the nearest integers, we’ll look at how close they are to their floor, the largest integer below. Put another way, we’ll throw away the integer parts and look at the decimal parts.

First a theorem:

For almost all x > 1, the sequence (xn) for n = 1, 2, 3, … is u.d. mod 1. [1]

Here “almost all” is a technical term meaning that the set of x‘s for which the statement above does not hold has Lebesgue measure zero. The abbreviation “u.d.” stands for “uniformly distributed.” A sequence uniformly distributed mod 1 if the fractional parts of the sequence are distributed like uniform random variables.

Even though the statement holds for almost all x, it’s hard to prove for particular values of x. And it’s easy to find particular values of x for which the theorem does not hold.

From [1]:

… it is interesting to note that one does not know whether sequences such as (en), (πn), or even ((3/2)n) are u.d. mod 1 or not.

Obviously powers of integers are not u.d. mod 1 because their fractional parts are all 0. And we’ve shown before that powers of the golden ratio and powers of the plastic constant are near integers, i.e. their fractional parts cluster near 0 and 1.

The curious part about the quote above is that it’s not clear whether powers of 3/2 are uniformly distributed mod 1. I wouldn’t expect powers of any rational number to be u.d. mod 1. Either my intuition was wrong, or it’s right but hasn’t been proved, at least not when [1] was written.

The next post will look at powers of 3/2 mod 1 and whether they appear to be uniformly distributed.

* * *

[1] Kuipers and Niederreiter, Uniform Distribution of Sequences

Plastic powers

Last week I wrote a blog post showing that powers of the golden ratio are nearly integers. Specifically, the distance from φn to the nearest integer decreases exponentially as n increases. Several people pointed out that the golden constant is a Pisot number, the general class of numbers whose powers are exponentially close to integers.

The so-called plastic constant P is another Pisot number, in fact the smallest Pisot number. P is the real root of x3x – 1 = 0.

P = \frac{ (9 - \sqrt{69})^{1/3} + (9 + \sqrt{69})^{1/3} }{ 2^{1/3} \,\,\, 3^{2/3} } = 1.3247\ldots

Because P is a Pisot number, we know that its powers will be close to integers, just like powers of the golden ratio, but the way they approach integers is more interesting. The convergence is slower and less regular.

We will the first few powers of P, first looking at the distance to the nearest integer on a linear scale, then looking at the absolute value of the distance on a logarithmic scale.

distance from powers of plastic constant to nearest integer

distance from powers of plastic constant to nearest integer, log scale

As a reminder, here’s what the corresponding plots looked like for the golden ratio.

distance from powers of golden ratio to nearest integer

distance from powers of golden ratio to nearest integer, log scale

Visualizing kinds of rings

When I first saw ring theory, my impression was that there were dozens of kinds of rings with dozens of special relations between them—more than I could keep up with. In reality, there just a few basic kinds of rings, and the relations between them are simple.

Here’s a diagram that shows the basic kinds of rings and the relations between them. (I’m only looking at commutative rings, and I assume ever ring has a multiplicative identity.)

Types of commutative rings

The solid lines are unconditional implications. The dashed line is a conditional implication.

  • Every field is a Euclidean domain.
  • Every Euclidean domain is a principal ideal domain (PID).
  • Every principal ideal domain is a unique factorization domain (UFD).
  • Every unique factorization domain is an integral domain.
  • A finite integral domain is a field.

Incidentally, the diagram has a sort of embedded pun: the implications form a circle, i.e. a ring.

More mathematical diagrams:

Golden powers are nearly integers

Nautilus, golden ratio

This morning I was reading Terry Tao’s overview of the work of Yves Meyer and ran across this line:

The powers φ, φ2, φ3, … of the golden ratio lie unexpectedly close to integers: for instance, φ11 = 199.005… is unusually close to 199.

I’d never heard that before, so I wrote a little code to see just how close golden powers are to integers.

Here’s a plot of the difference between φn and the nearest integer:

distance from powers of golden ratio to nearest integer

(Note that if you want to try this yourself, you need extended precision. Otherwise you’ll get strange numerical artifacts once φn is too large to represent exactly.)

By contrast, if we make the analogous plot replacing φ with π we see that the distance to the nearest integer looks like a uniform random variable:

distance from powers of pi to nearest integer

The distance from powers of φ to the nearest integer decreases so fast that cannot see it in the graph for moderate sized n, which suggests we plot the difference on the log scale. (In fact we plot the log of the absolute value of the difference since the difference could be negative and the log undefined.) Here’s what we get:

absolute distance from powers of golden ratio to nearest integer on log scale

After an initial rise, the curve is apparently a straight line on a log scale, i.e. the absolute distance to the nearest integer decreases almost exactly exponentially.

Related posts:

Duals and double duals of Banach spaces

The canonical examples of natural and unnatural transformations come from linear algebra, namely the relation between a vector space and its first and second duals. We will look briefly at the finite dimensional case, then concentrate on the infinite dimensional case.

Two finite-dimensional vector spaces over the same field are isomorphic if and only if they have the same dimension.

For a finite dimensional space V, its dual space V* is defined to be the vector space of linear functionals on V, that is, the set of linear functions from V to the underlying field. The space V* has the same dimension as V, and so the two spaces are isomorphic. You can do the same thing again, taking the dual of the dual, to get V**. This also has the same dimension, and so V is isomorphic to V** as well as V*. However, V is naturally isomorphic to V** but not to V*. That is, the transformation from V to V* is not natural.

Some things in linear algebra are easier to see in infinite dimensions, i.e. in Banach spaces. Distinctions that seem pedantic in finite dimensions clearly matter in infinite dimensions.

The category of Banach spaces considers linear spaces and continuous linear transformations between them. In a finite dimensional Euclidean space, all linear transformations are continuous, but in infinite dimensions a linear transformation is not necessarily continuous.

The dual of a Banach space V is the space of continuous linear functions on V. Now we can see examples of where not only is V* not naturally isomorphic to V, it’s not isomorphic at all.

For any real p > 1, let q be the number such that 1/p  + 1/q = 1. The Banach space Lp is defined to be the set of (equivalence classes of) Lebesgue integrable functions f such that the integral of |f|p is finite. The dual space of Lp is Lq. If p does not equal 2, then these two spaces are different. (If p does equal 2, then so does qL2 is a Hilbert space and its dual is indeed the same space.)

In the finite dimensional case, a vector space V is isomorphic to its second dual V**. In general, V can be embedded into V**, but V** might be a larger space. The embedding of V in V** is natural, both in the intuitive sense and in the formal sense of natural transformations, discussed in the previous post. We can turn an element of V into a linear functional on linear functions on V as follows.

Let v be an element of V and let f be an element of V*. The action of v on f is simply fv. That is, v acts on linear functions by letting them act on it!

This shows that some elements of V** come from evaluation at elements of V, but there could be more. Returning to the example of Lebesgue spaces above, the dual of L1 is L, the space of essentially bounded functions. But the dual of L is larger than L1. That is, one way to construct a continuous linear functional on bounded functions is to multiply them by an absolutely integrable function and integrate. But there are other ways to construct linear functionals on L.

A Banach space V is reflexive if the natural embedding of V in V** is an isomorphism. For p > 1, the spaces Lp are reflexive.

However, R. C. James proved the surprising result that there are Banach spaces that are isomorphic to their second duals, but not naturally. That is, there are spaces V where V is isomorphic to V**, but not via the natural embedding; the natural embedding of V into V** is not an isomorphism.

Related: Applied functional analysis

Natural transformations

The ladder of abstractions in category theory starts with categories, then functors, then natural transformations. Unfortunately, natural transformations don’t seem very natural when you first see the definition. This is ironic since the original motivation for developing category theory was to formalize the intuitive notion of a transformation being “natural.” Historically, functors were defined in order to define natural transformations, and categories were defined in order to define functors, just the opposite of the order in which they are introduced now.

A category is a collection of objects and arrows between objects. Usually these “arrows” are functions, but in general they don’t have to be.

A functor maps a category to another category. Since a category consists of objects and arrows, a functor maps objects to objects and arrows to arrows.

A natural transformation maps functors to functors. Sounds reasonable, but what does that mean?

You can think of a functor as a way to create a picture of one category inside another. Suppose you have some category and pick out two objects in that category, A and B, and suppose there is an arrow f between A and B. Then a functor F would take A and B and give you objects FA and FB in another category, and an arrow Ff between FA and FB. You could do the same with another functor G. So the objects A and B and the arrow between them in the first category have counterparts under the functors F and G in the new category as in the two diagrams below.

A natural transformation α between F and G is something that connects these two diagrams into one diagram that commutes.

The natural transformation α is a collection of arrows in the new category, one for every object in the original category. So we have an arrow αA for the object A and another arrow αB for the object B. These arrows are called the components of α at A and B respectively.

Note that the components of α depend on the objects A and B but not on the arrow f. If f represents any other arrow from A to B in the original category, the same arrows αA and αB fill in the diagram.

Natural transformations are meant to capture the idea that a transformation is “natural” in the sense of not depending on any arbitrary choices. If a transformation does depend on arbitrary choices, the arrows αA and αB would not be reusable but would have to change when f changes.

The next post will discuss the canonical examples of natural and unnatural transformations.

Related: Applied category theory

How areas of math are connected

In my previous post, I discussed how number theory and topology relate to other areas of math. Part of that was to show a couple diagrams from  Jean Dieudonné’s book Panorama of Pure Mathematics, as seen by N. Bourbaki. That book has only small star-shaped diagrams considering one area of math at a time. I’ve created a diagram that pastes these local views into one grand diagram. Along the way I’ve done a little editing because the original diagrams were not entirely consistent.

Here’s a condensed view of the graph. You can find the full image here.

The graph is so dense that it’s hard to tell which areas have the most or least connections. Here are some tables to clarify that. First, counting how many areas an particular area contributes to, i.e. number of outgoing arrows.

| Area                                | Contributions |
| Homological algebra                 |            12 |
| Lie groups                          |            11 |
| Algebraic and differential topology |            10 |
| Categories and sheaves              |             9 |
| Commutative algebra                 |             9 |
| Commutative harmonic analysis       |             9 |
| Algebraic geometry                  |             8 |
| Differential geometry and manifolds |             8 |
| Integration                         |             8 |
| Partial differential equations      |             8 |
| General algebra                     |             7 |
| Noncommutative harmonic analysis    |             6 |
| Ordinary differential equations     |             6 |
| Spectral theory of operators        |             6 |
| Analytic geometry                   |             5 |
| Automorphic and modular forms       |             5 |
| Classical analysis                  |             5 |
| Mathematical logic                  |             5 |
| Abstract groups                     |             4 |
| Ergodic theory                      |             4 |
| Probability theory                  |             4 |
| Topological vector spaces           |             4 |
| General topology                    |             3 |
| Number theory                       |             3 |
| Von Neumann algebras                |             2 |
| Set theory                          |             1 |

Next, counting the sources each area draws on, i.e. counting incoming arrows.

| Area                                | Sources |
| Algebraic geometry                  |      13 |
| Number theory                       |      12 |
| Lie groups                          |      11 |
| Noncommutative harmonic analysis    |      11 |
| Algebraic and differential topology |      10 |
| Analytic geometry                   |      10 |
| Automorphic and modular forms       |      10 |
| Ordinary differential equations     |      10 |
| Ergodic theory                      |       9 |
| Partial differential equations      |       9 |
| Abstract groups                     |       8 |
| Differential geometry and manifolds |       8 |
| Commutative algebra                 |       6 |
| Commutative harmonic analysis       |       6 |
| Probability theory                  |       5 |
| Categories and sheaves              |       4 |
| Homological algebra                 |       4 |
| Spectral theory of operators        |       4 |
| Von Neumann algebras                |       4 |
| General algebra                     |       2 |
| Mathematical logic                  |       1 |
| Set theory                          |       1 |
| Classical analysis                  |       0 |
| General topology                    |       0 |
| Integration                         |       0 |
| Topological vector spaces           |       0 |

Finally, connectedness, counting incoming and outgoing arrows.

| Area                                | Connections |
| Lie groups                          |          22 |
| Algebraic geometry                  |          21 |
| Algebraic and differential topology |          20 |
| Noncommutative harmonic analysis    |          17 |
| Partial differential equations      |          17 |
| Differential geometry and manifolds |          16 |
| Homological algebra                 |          16 |
| Ordinary differential equations     |          16 |
| Analytic geometry                   |          15 |
| Automorphic and modular forms       |          15 |
| Commutative algebra                 |          15 |
| Commutative harmonic analysis       |          15 |
| Number theory                       |          15 |
| Categories and sheaves              |          13 |
| Ergodic theory                      |          13 |
| Abstract groups                     |          12 |
| General algebra                     |          10 |
| Spectral theory of operators        |          10 |
| Probability theory                  |           9 |
| Integration                         |           8 |
| Mathematical logic                  |           6 |
| Von Neumann algebras                |           6 |
| Classical analysis                  |           5 |
| Topological vector spaces           |           4 |
| General topology                    |           3 |
| Set theory                          |           2 |

There are some real quirks here. The most foundational areas get short shrift. Set theory contributes to only one area of math?! Topological vector spaces don’t depend on anything, not even topology?!

I suspect Dieudonné had in mind fairly high-level contributions. Topological vector spaces, for example, obviously depend on topology, but not deeply. You could do research in the area while seldom drawing on more than an introductory topology course. Elementary logic and set theory are used everywhere, but most mathematicians have no need for advanced logic or set theory.

More math diagrams:

Mathematical balance of trade

Areas of math all draw on and contribute to each other. But there’s a sort of trade imbalance between areas. Some, like analytic number theory, are net importers. Others, like topology, are net exporters.

Analytic number theory uses the tools of analysis, especially complex analysis, to prove theorems about integers. The first time you see this it’s quite a surprise. But then you might expect that since analysis contributes to number theory, then number theory must contribute to analysis. But it doesn’t much.

Topology imports ideas from algebra. But it exports more than in imports, to algebra and to other areas. Topology started as a generalization of geometry. Along the way it developed ideas that extend far beyond geometry. For example, computer science, which ostensibly has nothing to do with whether you can continuously deform one shape into another, uses ideas from category theory that were developed initially for topology.

Here’s how Jean Dieudonné saw things. The following are my reconstructions of a couple diagrams from his book Panorama of Pure Mathematics, as seen by N. Bourbaki. An arrow from A to B means that A contributes to B, or B uses A.

For number theory, some of Dieudonné’s arrows go both ways, some only go into number theory. No arrows go only outward from number theory.

With topology, however, there’s a net flux out arrows going outward.

These diagrams are highly subjective. There’s plenty of room for disagreement. Dieudonné wrote his book 35 years ago, so you might argue that they were accurate at the time but need to be updated. In any case, the diagrams are interesting.

Update: See the next post of a larger diagram, sewing together little diagrams like the ones above.

Numerically integrating periodic functions

The trapezoid rule is the most obvious numerical integration technique. It comes directly from the definition of a definite integral, just a Riemann sum.

It’s a very crude technique in general; you can get much more accuracy with the same number of function evaluations by using a more sophisticated method. But for smooth periodic functions, the trapezoid rule works astonishingly well.

This post will look at two similar functions. The trapezoid rule will be very accurate for one but not for the other. The first function is

g(x) = exp( cos(x) ).

The second function, h(x) replaces the cosine with its Taylor approximation 1 – x2/2. That is,

h(x) = exp(1 – x2/2 ).

The graph below shows both functions.

Both functions are perfectly smooth. The function g is naturally periodic with period 2π. The function h could be modified to be a periodic function with the same period since h(-π) = h(π).

But the periodic extension of h is not smooth. It’s continuous, but it has a kink at odd multiples of π. The derivative is not continuous at these points. Here’s a close-up to show the kink.

Now suppose we want to integrate both functions from -π to π. Over that range both functions are smooth. But the behavior of h “off stage” effects the efficiency of the trapezoid rule. Making h periodic by pasting copies together that don’t match up smoothly does not make it act like a smooth periodic function as far as integration is concerned.

Here’s the error in the numerical integration using 2, 3, 4, …, 10 integration points.

The integration error for both functions decreases rapidly as we go from 2 to 5 integration points. And in fact the integration error for h is slightly less than that for g with 5 integration points. But the convergence for h practically stops at that point compared to g where the integration error decreases exponentially. Using only 10 integration points, the error has dropped to approximately 7×10-8 while the error for h is five orders of magnitude larger.

Related: Numerical integration consulting