# Monthly Archives: August 2012

## The danger of confusing cosets and numbers

As I mentioned yesterday, I’d like to consider a proposed proof of the Goldbach Conjecture that has garnered some attention, at least some attention from people who ask me about things like the validity of proofs of the Goldbach Conjecture. I like this in particular because it illustrates how I look through some papers (those towards which I’m a bit skeptical) and it illustrates a problem I’ve seen before: switching between interpreting a number as an element of the integers and an element of $\mathbb{Z}/n\mathbb{Z}$. (There is a certain problem with this, in that although I ‘do number theory,’ were the conjecture proved it is almost certain that I would be not at all familiar with the methods of proof).
In particular, I’ll be looking at the 19 August 2012 preprint “The Goldbach’s conjecture proved” by Agostino Prastaro (the pdf is here). The rest after the fold –

Posted in Math.NT, Mathematics, Open | | 4 Comments

## An elementary proof of when 2 is a quadratic residue

This has been a week of asking and answering questions from emails, as far as I can see. I want to respond to two questions publicly, though, as they’ve some interesting value. So this is the first of a pair of blog posts. One is a short and sweet elementary proof of when $2$ is a quadratic residue of a prime, responding to Moschops’s comments on an earlier blog post. But to continue my theme of some good and some bad, I’d also like to consider the latest “proof” of the Goldbach conjecture (which I’ll talk about in the next post tomorrow). More after the fold:

Posted in Expository, Math.NT, Mathematics, SE | | 7 Comments

## Dancing ones PhD

In my dealings with the internet this week, I am reminded of a quote by William Arthur Ward, the professional inspirator:

We can throw stones, complain about them, stumble on them, climb over them, or build with them.

In particular, I have been notified by two different math-related things. Firstly, most importantly and more interestingly, my friend Diana Davis created a video entry for the “Dance your PhD” contest. It’s aboutÂ Cutting Sequences on the Double Pentagon, and you can (and should) look at it on vimeo. It may even be the first math dance-your-PhD entry! You might even notice that I’m in the video, and am even waving madly (I had thought it surreptitious at the time) around 3:35.

That’s the positive one, the “Building with the Internet,” a creative use of the now-common-commodity. After the fold is the travesty.

Posted in Humor, Story | Tagged , , | 5 Comments

## Precalculus Supplement: Synthetic Division

I think it is a sign.

In the question How does Synthetic Division Work? by the user Riddler on math.stackexchange, Riddler says that he’s never seen a proof of Synthetic Division. This gave me a great case of Mom’s Corollary (the generalization of the fact that when mothers tell you something, you are often reminded of specific cases of it within three days – at least with my mom), as it came up with a student whom I’m tutoring. It turns out many of my students haven’t liked synthetic division. I chatted with some of the other Brown grads, and in general, they didn’t like synthetic division either.

It was one of those things that was taught to us before we thought about why different things worked. Somehow, it wasn’t interesting or useful enough to be revisited. But let’s take a look at synthetic division after the fold:

Posted in Brown University, Mathematics, precalculus | | 3 Comments