## Eratosuite

December 15, 2015

The other day in class, we were looking at recurrence relations and how to solve them with the characteristic equation. Among the examples I gave was a recurrence that seemed to give a good number of primes numbers. Are there many of those? How do we find them?

Well, we need two things: a method of testing if a given number is prime, and a method of generating recurrences—parameters and initial conditions. Well, make that three: we also need to generate the suite generated by the recurrence relation. Let’s go!

## Pythagorean Primes

December 1, 2015

Last Week, we had a look at Pythagorean triples. Remember: a Pythagorean triple is three natural numbers (positive integers) such that $a^2+b^2=c^2$. Most of the times, even with $a$ and $b$ natural numbers, $c$ is irrational. Sometimes $c$ is prime; sometimes $c^2$ is. Is either frequent?

## Hash Table and Magic (Relatively Primes) Numbers

September 17, 2013

Today, let’s talk about hash tables. Or, more precisely, one type of secondary probing technique, one that uses some number theory—but not quadratic residues, just relatively prime numbers.

I don’t know if you’re like me, but it often bugs me when I read something in a textbook that seems to make sense, but is claimed without proof nor demonstration (oh, I think I said that before). That seems to happen particularly often when I read stuff about data structures, and this time I decided to explore one of those claims.

## Euclid and Primality Testing (III)

July 2, 2013

So in previous installments, we looked at how to use the euclidean algorithm to devise a Las Vegas-type algorithm for primality testing. We also found that, in general, simply testing factors one after the other is much more efficient (but that doesn’t mean that there are not better algorithms to test for primality!).

We also considered only relatively small primorials (the products of the first $n$ prime numbers) since they rapidly exceeded $2^{32}$. But just how fast do primorials grow?

## Euclid, Primes numbers, and a Bit of Algorithm Analysis

June 25, 2013

Last time we had a look at using the greatest common divisor and Euclid’s algorithm to devise a Las Vegas algorithm for primality testing. We also had a look at how the inclusion exclusion principle helps us determine the proportion of the numbers correctly tested.

However, we finished by asking ourselves if the method is actually efficient compared to, say, simply testing small divisors, one by one. Let us now find out.

## Generating Random Sequences (part I)

September 29, 2009

Every once in a while, we need a random sequence. Whether to test a data structure’s performance or to run probabilistic unit tests, the provided rand primitive from your favorite programming language has several limitations. First, it’s been known for a while that if most implementations of the C standard library rand() function are not very random, despite being “good enough” in great many cases. Second, and more importantly, it does not allow you to easily control the period nor to generate a permutation on $0\ldots n-1$, for example.

There are many methods of generating pseudo-random number sequences. Not all exhibit the same properties and, accordingly, a method may be more useful in one case and perfectly useless in another. High quality pseudo-random number generation is a notoriously hard endeavor, but there is a number of very simple algorithms that will get you out of trouble for certain specific tasks. Let us consider, for example, the example where the pseudo-random generator must generate the numbers in $0\ldots{}n-1$ exactly once, in a random order, of course, in exactly $n$ draws.