A dense version of Kruskal's algorithm

In this post I propose a dense version of Kruskal’s algorithm that I have not seen in the literature. It has a runtime of $$\Theta(n^2)$$, which is better than $$O(m \log m)$$, the runtime of the standard version of the algorithm, when $$m \in \Theta(n^2)$$.

Experimentation shows the runtime of a simple implementation to be similar to Prim’s algorithm for the dense case, and a marked improvement over the usual, sparse version of Kruskal.

AVL trees are data structures for ordered element storage and retrieval. They are a common implementation of sets and maps. The advantage they provide over regular binary search trees is they bound the height of the tree, resulting in logarithmic insertion, removal, and retrieval, and linear traversal.

In this post we will see how one can implement such a structure in a strongly typed language - in particular, Haskell - with one important note: the height invariant of this tree will be known to hold at compile time.

Fast modular Fibonacci

It’s relatively frequent in programming competitions that one needs to compute large terms of a linear recurrence with constant coefficients, like the Fibonacci sequence, modulo some number.

In this post we’ll see a bit about how to compute these terms fast, exploring not only the programming side, but the mathematics behind it that makes it work.

Generalized rock-paper-scissors, best-of-3

In this post I analyze a generalized version of generalized version of rock-paper-scissors, to any $$2n+1$$ choices, $$n \ge 1$$. I also look at a specific playing rule, which is “best-of-three”, meaning the two players play until one has two victories. I then generalize this to $$k$$ victories, $$k \ge 1$$.

Spectral clustering

In this post I introduce the notion of spectral clustering, and link to a small C++ impementation. I discuss the formalism and mathematical motivation for it as a relaxation of an NP-hard problem, and finally show an example of its behavior on a typical dataset on which $$k$$-means has bad behavior.

The boolean satisfiability problem

The boolean satisfiability problem, or SAT in computational complexity jargon, acts as a cornerstone of the NP complexity class. For every $$k \in \mathbb{N}$$, $$k$$-SAT is a restriction of SAT where each clause has exactly $$k$$ terms.

In this post I analyze the boundary between P and NP that occurs as we increase $$k$$ in $$k$$-SAT, for $$1 \le k \le 3$$. I also prove that $$3$$-SAT is already NP-complete.

Braess's paradox and the price of anarchy

Braess’ paradox is the notion that adding free edges to a network where traffic flows through with a given cost per edge, may actually increase the average cost to flow through the network. This counterintuitive result is based on Game Theory, since it can be modelled as a suboptimal Nash equilibrium.

In this post I show a classical example of it, and I introduce the notion of the price of anarchy.

An introduction to incidence matrices

In the third and last graph representation post, I discuss another linear algebraic representation of a graph, the incidence matrix. I also show some theorems which are easily proved using this representation.