On Thursday 29 August 2019 I gave a talk about expansivity. I focused on positive expansivity and discussed a general statement which has a most remarkable consequence for cellular automata theory.
Find the talk on my personal blog HERE
Today, the 1st of March 2018, I gave what ended up being the first of a series of Theory Lunch talks about subadditive functions. The idea is to give an introduction to the subject, following Hille’s and Lind and Marcus’s textbooks, and stating an important theorem by the Hungarian mathematician Mihály Fekete; then, discuss some extensions to the case of many variables and their implications in the theory of cellular automata, referring to two of my papers from 2008, one of them with Tommaso Toffoli and Patrizia Mentrasti.
Let’s start from the beginning: Continue reading
Today, the 8th of September 2016, we had a very interesting discussion about a theorem, due to Selim G. Akl, pointed to me in a tweet by Andy Adamatzky. Such theorem has, according to Akl, the consequence that the Church-Turing thesis, a basic tenet of theoretical computer science, is false. Of course, surprising statements require solid arguments: is Akl’s solid enough?
First of all, let us recall what the Church-Turing thesis is, and what it is not. Its statement, as reported by the Stanford Encyclopedia of Philosophy, goes as follows: Continue reading
First-order formal logic is a standard topic in computer science. Not so for second-order logic: which, though used the default in fields of mathematics such as topology and analysis, is usually not treated in standard courses in mathematical logic. For today’s Theory Lunch I discussed some classical theorems that hold for first-order logic, but not for second-order logic: Continue reading
On Thursday, the 25th of March 2015, Venanzio Capretta gave a Theory Lunch talk about Goodstein’s theorem. Later, on the 9th of March, Wolfgang Jeltsch talked about ordinal numbers, which are at the base of Goodstein’s proof. Here, I am writing down a small recollection of their arguments.
Given a base , consider the base- writing of the nonnegative integer
where each is an integer between and . The Cantor base- writing of is obtained by iteratively applying the base- writing to the exponents as well, until the only values appearing are integers between and . For example, for and , we have
Given a nonnegative integer , consider the Goodstein sequence defined for by putting , and by constructing from as follows: Continue reading
At today’s Theory Lunch I discussed limit languages of cellular automata, and Lyman Hurd’s example of a CA whose limit language is not regular. I wrote about this on my other blog.