14 May 2009

Square roots and sunscreen

Also, here's an interesting tidbit, from this New York Times piece on SPF. SPF, or "sun protection factor", is the number on the sunscreen bottle; if a properly applied sunscreen lets through a fraction p of the UV rays it's meant to protect against, then that sunscreen has SPF 1/p. (The numbers in the article talk about the proportion of the UV rays which are blocked; in this case, if a fraction q of the UV rays are blocked, the sunscreen has SPF 1/(1-q).)

Anyway, you're supposed to apply some ridiculous amount of sunscreen to your body, about an ounce. This seems like a lot to most people, because that stuff is expensive! So a lot of people underapply sunscreen. (I'll include myself here.) The article quotes Darrell Rigel, NYU dermatologist, as saying that if you apply half the sunscreen you're "supposed" to, you have to take the square root of the SPF.

That sounds obvious once you think about it -- but I'll admit I'd never thought about it. Say I have a sunscreen that allows through one-sixteenth of the light which hits it when applied properly. Now imagine splitting it up into two coats, each of which allows through the same proportion of the light that hits it. One-fourth of the light makes it through the outer coat; one-fourth of that light makes it through to the skin.

Of course there are issues with this analysis, but according to this paper in the British Journal of Dermatology it appears to hold up. And applying twice the usual amount of sunscreen apparently squares the SPF. (The effect is actually a bit less than this, because sunscreens don't block all wavelengths equally, nor does the sun's spectrum contain all wavelengths equally.)

This all implies that if you want to compare prices of sunscreens, you should divide the cost of the sunscreen by the product of the bottle's volume and the logarithm of the SPF. Do sunscreen prices actually work this way?

Twitter Ratio - why?

Twitter Ratio calculates the ratio of the number of followers you have on Twitter to the number of people who follow you.

Yes, there's a web site to do division. (And Twitter reports the two numbers involved in the quotient, so it's not even like this web site is doing the counting.)

Apparently it also saves historical numbers, so it's not entirely worthless, but it still seems like an odd thing to base a site around.

08 May 2009

The third derivative of the employment rate is positive

The third derivative of the number of people employed in the United States is positive. (From 538.)

Nate Silver puts it as "the second derivative has improved", but let's face it, this is really a statement about the third derivative. Compare Nixon's 1972 statement that the rate of increase of inflation was decreasing, which Hugo Rossi pointed out in the Notices was a statement about the third derivative. (I seem to recall John Allen Paulos pointing this out in one of his books, but I don't recall which book and therefore can't date it relative to Rossi's letter in the Notices.)

The physics of singing in the shower

I was singing in the shower, as I do.

I noticed that certain notes seemed to resonate with the shower more than others.

These were, in ascending order, Eb2, G2, C3, and G3, where C4 is middle C. (These may not be exactly right; I don't have perfect pitch. The intervals are right, though.)

Exercise for the reader: how large is my shower?

07 May 2009

The Calkin-Wilf tree on Wikipedia

The Calkin-Wilf tree now has a Wikipedia page. This is an infinite binary tree with rational numbers at the nodes, such that it contains each rational number exactly once. In the sequence of rational numbers that one gets from breadth-first traversal of the tree,

1/1, 1/2,2/1, 1/3, 3/2, 2/3, 3/1, 1/4, 4/3, 3/5, 5/2, 2/5, 5/3, 3/4, 4/1, ...

the denominator of each number is the numerator of the next; furthermore the sequence of denominators (or of numerators) actually counts something. Plus, there are some interesting pictures that come from plotting these sequences, and some interesting probabilistic properties (see arXiv:0801.0054 for some of the probabilistic stuff, although I actually just found it and haven't read it thoroughly) I've given a talk about this; one day I'll write down some version of it. This is one of my favorite mathematical objects.

It looks like we've got David Eppstein to thank for this. It was introduced in this article by Calkin and Wilf.

04 May 2009

Bears, pigs, and the like

The blog's been slow. I've been off writing real mathematics, thinking for and preparing for the class I'm teaching this summer, and so on. But I'm still here!

And while I'm here, you should read Chad Orzel on the faulty thermodynamics of children's stories. In the story of Goldilocks and the three bears, one would expect that the papa bear is the largest, then the mama bear, and then the baby bear. Furthermore, you'd think that the larger the bear, the larger the bowl of porridge, and the slower it should cool off. But it doesn't seem to work that way! Read the comments come up with some interesting explanations.

Exercise for the scientifically-inclined reader: comment on the physical implications of the Three Little Pigs.

Exercise for the not-so-scientifically-inclined reader: what's with all the animals coming in threes?