We at CatSynth are delighted to be hosting the 35th Carnival of Mathematics.

The date “Friday the 13th” of course evokes many superstitions. And perhaps that is why we have so few entries so far this weekend. That having been said, we at CatSynth take a dim view of superstitions. Many of the traditions that led to the irrational fear of the number 13 (including skipping over of 13 in building floor numbers, etc) are the same ones that promote the unfair treatment of black cats. Neither has a place in a modern, rational, society.

Consider that 13th of the month is no more likely to fall on a Friday than any other day of the week. Over a period of 28 years, that is 48 “Fridays the 13th,” as described here.

Let us turn away from superstition and myth, so some of the more mathematical properties of the number 13. It is a prime number (indeed, the sixth smallest prime). It is also a well-known twin prime, and it is one of only three known Wilson primes. For those less interested that I am in prime numbers, it is also a Fibnoacci number, and the number of Archimedean solids.

You can find more about the mathematics and mythology of 13 at “13 the Unlucky”.

Last week’s host, the blog 360, has a post about 13 and other “unlucky” numbers. But they also contribute a demonstration, using Godzilla, of how to calculate the speed of a

dinosaur by its footprints.

**Recursivity** introduces us to the Open Problem Garden, a site collecting open problems in mathematics and theoretical computer science. He presents an open problem about discrete iterations that is easy to state yet frustratingly hard to solve, as are many of the great problems of mathematics.

Looking for a more modest challenge? **Math and Logic Play** presents a bicycle-racing puzzle. Practice your math skills while staying healthy and reducing your carbon footprint…

**Larry Ferlazzo** describes a site for math games and education in Tutpup Math & Spelling Games.

**AndrĂ©e** from **meeyauw** treats us to Sierpinski cookies and earings this week. Those cookies look pretty good, but challenging to make. Fractals are by nature very detail oriented, and I am by nature very not detail oriented. But I’m sure they would taste great in either case.

The state of math education (in the United States) in a frequent topic on Carnival of Mathematics. This week, **female math professor** critiques an elementary school math program at **Life on the Tenure-Track**. It seems that at least in some classes, teachers aren’t able to make it through the full syllabus.

More topics in math education can be found at Teach College Math, where we are introduced to the **Math Girl** videos. “Who says there’s nothing good on YouTube?”

For the more discriminating reader, we have Discriminating Determinants, third in a series from **Matt-a-matical Thinking**.

We get back to our theme of the number 13. The **fashionablemathematician** presents the sum of prime factors. For a prime number like **13**, the sum is of course the same: 13. But are there other numbers whose prime factors sum to 13? Well, the number **22** has prime factors **2** and **11**, which add up to 13.

Sometimes a mathematician can end up being an astrophysicist, especially on Friday the 13th, as is the case with this article from Math Trek.

That is all the posts we currently have. We will continue to add sites throughout the weekend, so please leave a comment or contact us if you would like to participate.

]]