John D. Cook

Not all messages encrypted with the RSA algorithm can be decrypted. This post will show why this is possible and why it does not matter in practice. RSA in a nutshell RSA encryption starts by finding two large primes, p and q. These primes are kept secret, but their productn = pq is made public. [...]The post RSA encrypted messages that cannot be decrypted first appeared on John D. Cook.

Dimension 2 The equation for the perimeter of an ellipse is wherea is the semimajor axis, e is eccentricity, and E is a special function. The equation is simple, in the sense that it has few terms, but it is not elementary, because it depends on an advanced function, the complete elliptic integral of the [...]The post Hyperellipsoid surface area first appeared on John D. Cook.

I posted some notes this morning on how to find the perimeter of an ellipse given its axes. The notes include a simple approximation, a better but more complicated approximation, and the exact value. So given the semi axes a and b, the notes give three ways to compute the perimeter p. If you are [...]The post Solve for ellipse axes given perimeter first appeared on John D. Cook.

The home team lost in a new way yesterday. The Baltimore Ravens beat the Houston Texans by 25-9. This was the first time that score has been seen in the NFL. Possible individual team scores How many scores are possible? It is possible to score any number of points except 1. You can score 2 [...]The post Possible and actual football scores first appeared on John D. Cook.

Earlier I wrote a post showing what happens when you start with an equilateral triangle, then repeatedly subdivide it into smaller and smaller triangles by drawing lines from the centroid (barycenter) to each of the vertices. I mentioned in that post that I moved the code for finding the center to its own function because [...]The post How you define center matters a lot first appeared on John D. Cook.

I just saw a tweet from Dave Richeson saying I remember as a kid calculating the size difference (diameter) of a belt between each hole. Now I think about it every time I wear a belt. Holes 1 inch apart change the diameter by about one-third of an inch (1/). [Assuming people have a circular [...]The post Belt around an elliptical waist first appeared on John D. Cook.

The other day I saw where Cliff Pickover tweeted some images of triangles recursively subdivided by adding a point to the barycenter of each triangle. The images were not what I expected, so I wanted to reproduce the images to look into this further. Here are the first three steps: I set the alpha value [...]The post Recursive triangle subdivision first appeared on John D. Cook.

One of the most common things a statistician is asked to do is compute a sample. There are well known formulas for this, so why isn't calculating a sample size trivial? As with most things in statistics, plugging numbers into a formula is not the hard part. The hard part is deciding what numbers to [...]The post Justifiable sample size first appeared on John D. Cook.

A large class of checksum algorithms have the following pattern: Think of the bits in a file as the coefficients in a polynomial P(x). Divide P(x) by a fixed polynomial Q(x) mod 2 and keep the remainder. Report the remainder as a sequence of bits. In practice there's a little more to the algorithm than [...]The post Checksum polynomials first appeared on John D. Cook.

Given a square complex matrix A, the Jordan normal form of A is a matrix J such that and J has a particular form. The eigenvalues of A are along the diagonal of J, and the elements above the diagonal are 0s or 1s. There's a particular pattern to the 1s, giving the matrix J [...]The post Jordan normal form: 1's above or below diagonal? first appeared on John D. Cook.

When is the discrete Fourier transform of a vector proportional to the original vector? And when that happens, what is the proportionality constant? In more formal language, what can we say about the eigenvectors and eigenvalues of the DFT matrix? Setup I mentioned in the previous post that Mathematica's default convention for defining the DFT [...]The post Eigenvectors of the DFT matrix first appeared on John D. Cook.

Just as there are multiple conventions for defining the Fourier transform, there are multiple conventions for defining the discrete Fourier transform (DFT), better known as the fast Fourier transform (FFT). [1] This post will look at two DFT conventions, one used in Python's NumPy library, and one used in Mathematica. There are more conventions in [...]The post DFT conventions: NumPy vs Mathematica first appeared on John D. Cook.

Math books often use some illustration from the book contents as cover art. When they do, there's often some mystery to the cover art, and a sense of accomplishment when you get far enough into the book to understand the significance of the cover. (See examples here.) William L. Briggs and Van Emden Henson wrote [...]The post DFT mandalas first appeared on John D. Cook.

In his book How to Read Literature Like a Professor, Thomas Foster says that if a poem looks like a square on the printed page, it's likely a sonnet. The miracle of the sonnet, you see, is that it is fourteen lines long and written almost always in iambic pentameter. ... suffice it to say [...]The post Sonnets are square first appeared on John D. Cook.

Suppose you've been monitoring a rare event for a long time, then you see your first occurrence on the Nth observation. Now what would you say about the event's probability? For example, suppose you're wondering whether dogs ever have two tails. You observe thousands of dogs and never see two tails. But then you see [...]The post First time seeing a rare event first appeared on John D. Cook.

Imagine the following dialog. Logarithms are usually taken to integer bases, like 2 or 10." What about e?" OK, that's an example of an irrational base, but it's the only one." Decibels are logarithms tobase 101/10." Really?!" Yeah, you can read about this here." That's weird. But logarithms are always take to bases bigger than [...]The post Stellar magnitude first appeared on John D. Cook.

US telephone area codes are allocated somewhat randomly. There was a deliberate effort to keep geographical proximity from corresponding to numerical proximity, unlike zip codes. (More of zip code proximity here.) In particular, consecutive area codes should belong to different states. The thought was that this would reduce errors. It's still mostly the case that [...]The post Area codes first appeared on John D. Cook.

I was flipping through Gravitation [1] this weekend and was curious about an illustration on page 309. This post reproduces that graph. The graph is centered at Cairo, Egypt and includes triangles whose side lengths are the distances between cities. The triangles are calculated using only distances, not by measuring angles per se. The geometry [...]The post Curvature at Cairo first appeared on John D. Cook.

Given the equations for two circles, how can you tell whether they intersect? And if they do intersect, how do you find the point(s) of intersection? MathWorld gives a derivation, but I'd like to add the derivation there in two ways. First, I'd like to be more explicit about the number of solutions. Second, I'd [...]The post Calculating the intersection of two circles first appeared on John D. Cook.

Paul Graham said Programming languages teach you not to want what they don't provide." He meant that as a negative: programmers using less expressive languages don't know what they're missing. But you could also take that as a positive: using a simple language can teach you that you don't need features you thought you needed. [...]The post A small programming language first appeared on John D. Cook.

Many numerical integration formulas over a finite interval have the form That is, the integral on the left can be approximated by evaluating the integrand f at particular nodes and taking the weighted sum, and the error is some multiple of a derivative of f evaluated at a point in the interval [a, b]. This [...]The post Quadrature rules and an impossibility theorem first appeared on John D. Cook.

In his book The World Beyond Your Head Matthew Crawford talks about jigs literally and metaphorically. A jig in carpentry is something to hold parts in place, such as aligning boards that need to be cut to the same length. Crawford uses the term more generally to describe labor-saving (or more importantly, thought-saving) techniques in [...]The post Jigs first appeared on John D. Cook.

This morning I wrote about Dan Piponi's fake prime function. This evening I thought about it again and wondered whether the chi-squared test could tell the difference between the distribution of digits in real primes and fake primes. If the distributions were obviously different, this would be apparent from looking at histograms. When distributions are [...]The post Can the chi squared test detect fake primes? first appeared on John D. Cook.

I have an account on Mastodon: johndcook@mathstodon.xyz. Note that's @math... and not @mast... One advantage to Mastodon is that you can browse content there without logging, while Twitter is becoming more of a walled garden. You can browse my account, for example, by going to the URL https://mathstodon.xyz/@johndcook There's hardly any content there at this [...]The post Mastodon account first appeared on John D. Cook.

Someone asked on Math Overflow about the distribution of digits in primes. It seems 0 is the least common digit and 1 the most common digit. Dan Piponi replies this is probably just a combination of general properties of sets of numbers with a density similar to the primes and the fact that primes end [...]The post Fake primes first appeared on John D. Cook.

I've been thinking about things that were ruined by doing about 5% more than was necessary, like an actor whose plastic surgery looks plastic. Sometimes excellence requires pushing ourselves to do more than we want to do or more than we think we can do. But sometimes excellence requires restraint. Context is everything. A few [...]The post Do 5% less first appeared on John D. Cook.

Suppose you are in an air traffic control tower observing a plane moving in a straight line and you want to rotate your frame of reference to align with the plane. In the new frame the plane is moving along a coordinate axis with no component of motion in the other directions. You could do [...]The post Tracking and the Euler rotation theorem first appeared on John D. Cook.

NLP software infers parts of speech by context. For example, the SpaCy NLP software can determine the parts of speech in the poem Jabberwocky even though the words are nonsense. More on this here. If you want to tell the parts of speech for isolated words, maybe software like SpaCy isn't the right tool. You [...]The post Using WordNet to create a PAO system first appeared on John D. Cook.

The Major mnemonic system is a method of converting numbers to words that can be more easily memorized. The basics of the system can be written on an index card, but there are practical details that are seldom written down. Presentations of the Major system can be misleading, intentionally or unintentionally, by implying that it [...]The post Memorizing four-digit numbers first appeared on John D. Cook.

Let A be an n * n complex matrix. The numerical range of A is the image of x*Ax over the unit sphere. That is, the numerical range of A is the set W(A) in defined by W(A) = {x*Ax | x n and ||x|| = 1} where x* is the conjugate transpose of [...]The post The numerical range ellipse first appeared on John D. Cook.

Ben Grimmer posted something yesterday on Twitter: A nice mathematical puzzle If you take a 4-norm ball and cut it carefully, you will find a two-norm ball. 3D printed visual evidence below. The puzzle: Why does this happen and how much more generally does it happen? (This question was first posed to me by Pablo [...]The post Random slices of a sphube first appeared on John D. Cook.

Are there more twin stars or twin primes? If the twin prime conjecture is true, there are an infinite number of twin primes, and that would settle the question. We don't know whether there are infinitely many twin primes, and it's a little challenging to find any results on how many twin primes we're sure [...]The post Twin stars and twin primes first appeared on John D. Cook.

Evenly placing points on a sphere is a difficult problem. It's impossible in general, and so you distribute the points as evenly as you can. The results vary according to how you measure how evenly the points are spread. However, there is a fast and simple way to distribute points that may be good enough, [...]The post Simple way to distribute points on a sphere first appeared on John D. Cook.

If you've only seen one definition of spherical coordinates, you may be shocked to discover that there are multiple conventions. In particular, mathematicians and geoscientists have different conventions. As Volker Michel put it in book on constructive approximation, Many mathematicians have faced weird jigsaw puzzles with misplaced continents after using a data set from a [...]The post Spherical coordinate Rosetta Stone first appeared on John D. Cook.

A Traveling Salesman tour visits a list of destinations using the shortest path. There's an obvious way to find the shortest path connecting N points: try all N! paths and see which one is shortest. Unfortunately, that might take a while. Texas has 254 counties, and so calculating a tour of Texas counties by brute [...]The post Creating a Traveling Salesman Tour of Texas with Mathematica first appeared on John D. Cook.

A spherical cap is the portion of a sphere above some horizontal plane. For example, the polar ice cap of the earth is the region above some latitude. I mentioned in this post that the area above a latitude is where R is the earth's radius. Latitude is the angle up from the equator. [...]The post Area and volume of hypersphere cap first appeared on John D. Cook.

In high dimensions, randomly chosen vectors are very likely nearly orthogonal. I'll unpack this a little bit then demonstrate it by simulation. Then I'll look at what happens when we restrict our attention to points with positive coordinates. *** The lengths of vectors don't contribute to the angles between them, so we may as well [...]The post Random points in a high-dimensional orthant first appeared on John D. Cook.

The previous post looked at cosine similarity for embeddings of words in vector spaces. Word embeddings like word2vec map words into high-dimensional vector spaces in such a way that related words correspond to vectors that are roughly parallel. Ideally the more similar the words, the smaller the angle between their corresponding vectors. The cosine similarity [...]The post Cosine similarity does not satisfy the triangle inequality first appeared on John D. Cook.

Natural language processing represents words as high-dimensional vectors, on the order of 100 dimensions. For example, the glove-wiki-gigaword-50 set of word vectors contains 50-dimensional vectors, and the the glove-wiki-gigaword-200 set of word vectors contains 200-dimensional vectors. The intent is to represent words in such a way that the angle between vectors is related to similarity [...]The post Angles between words first appeared on John D. Cook.

This post will discuss two scripting languages, but that's not what the post is really about. It's really about expressiveness and (or versus) productivity. *** I was excited to discover the awk programming language sometime in college because I had not used a scripting language before. Compared to C, awk was high-level luxury. Then a [...]The post Productive constraints first appeared on John D. Cook.

A Mobius transformation is a function of the form where ad - bc = 1. We usually think of z as a complex number, but it doesn't have to be. We could define Mobius transformations in any context where we can multiply, add, and divide, i.e. over any field. In particular, we could work over [...]The post Mobius transformations over a finite field first appeared on John D. Cook.

A common idiom in command line processing of text files is ... | sort | uniq | ... Some process produces lines of text. You want to pipe that text through sort to sort the lines in alphabetical order, then pass it to uniq to filter out all but the unique lines. The uniq utility [...]The post Sort and remove duplicates first appeared on John D. Cook.

The previous post looked at the swish function and related activation functions for deep neural networks designed to address the dying ReLU problem." Unlike many activation functions, the function f(x) is not monotone but has a minimum near x0 = -1.2784. The exact location of the minimum is where W is the Lambert W function, [...]The post Swish function and a Swiss mathematician first appeared on John D. Cook.

Swish, mish, and serf are neural net activation functions. The names are fun to say, but more importantly the functions have been shown to improve neural network performance by solving the dying ReLU problem." This happens when a large number of node weights become zero during training and do not contribute further to the learning [...]The post Swish, mish, and serf first appeared on John D. Cook.

In principle you generate an RSA key by finding two large prime numbers, p and q, and computing n = pq. You could, for example, generate random numbers by rolling dice, then type the numbers into Mathematica to test each for primaility until you find a couple prime numbers of the right size. In practice [...]The post Generating and inspecting an RSA private key first appeared on John D. Cook.

At its core, RSA encryption is modular exponentiation. That is, given a message m, the encrypted form of m is x = me mod n where e is a publicly known exponent and n is a product of two large primes. The number n is made public but only the holder of the private key [...]The post RSA encryption in practice first appeared on John D. Cook.

A few days ago I wrote about using the CMU Pronouncing Dictionary to search for words that decode to certain numbers in the Major mnemonic system. You can find a brief description of the Major system in that post. As large as the CMU dictionary is, it did not contain words mapping to some three-digit [...]The post Code to convert words to Major system numbers first appeared on John D. Cook.

The Allee effect is named after Warder Clyde Allee who added a term to the famous logistic equation. His added term is highlighted in blue. Here N is the population of a species over time, r is the intrinsic rate of increase, K is the carrying capacity, and A is the critical point. If you [...]The post Software and the Allee effect first appeared on John D. Cook.

That's a solved problem. So nobody knows how to solve it anymore." Once a problem is deemed solved" interest in the problem plummets. Solved" problems may not be fully solved, but sufficiently solved that the problem is no longer fashionable. Practical issues remain, but interest moves elsewhere. The software written for the problem slowly decays. [...]The post Solved problems becoming unsolved first appeared on John D. Cook.

There's an old saying The cobbler's son has no shoes." It's generally taken to mean that we can neglect to do for ourselves something we do for other people. I've been writing a few scripts for my personal use, things I've long intended to do but only recently got around to doing. I said something [...]The post The cobbler's son first appeared on John D. Cook.