Now to the actual definition of Horn clause. First, some standard logical terminology. A term is simply an expression built out of variables and function symbols. For example, x y-1 z is a term in the language of groups. An atomic formula is a...

My normal tendency is to write long posts that I never finish. I’ll start off this series with small posts to see if I can break the habit. The idea of Horn clauses emerged from model theory, so I will begin there. Model theory considers ideas that...

I was musing about the foundations of mathematics the other day, when it occurred to me that you could make a pretty good case that the key foundational idea of mathematics is that of Horn clauses (also known as universal Horn sentences). Horn...

J. P. May’s book, A Concise Course in Algebraic Topology, is available for download on his homepage. The book provides an overview of classical algebraic topology: homology and homotopy groups, K-theory, and cobordism.

I don’t normally link to sites that require registration, such as the New York Times, but <a href="http://fifthdown.blogs.nytimes.com/2010/01/01/week-17-matchups/"this preview of the final week of the NFL regular season features both a reference to...