and family trees.

The last blog I excitedly pointed out here was Transect Points, Philip Small's soil science blog. So you see, I give good blog advice. I'd also like you to consider bit-player, the personal blog of Brian Hayes who writes the Computing Science column for American Scientist. See Library Daze for a taste.

. . . at the end of my junior year in high school, I told my parents I was going to the beach for the summer, a fib I had concocted so they wouldn’t worry about me; I actually ran away to the Van Pelt Library of the University of Pennsylvania, where I wrote a novel about a kid who goes to the beach for the summer. (Yes, I know, what a waste! Next time I’ll write about a kid who goes to the library for the summer.)

But don't miss 0.203188.

In a “Computing Science” column titled “Rumours and Errours,” not quite a year ago, a leading role went to the nondescript number 0.203188. That number emerged from a simulation of how rumors spread through a society; given certain assumptions, 0.203188 is the proportion of the population that never hears the rumor.

A few weeks ago Paul Krapivsky of Boston University wrote me to say that the number mentioned in the column looked familiar, and he directed me to two other papers in which it appears . . . Both of those articles take up questions of human ancestry and genealogy, a topic that might seem remote from rumor-mongering, but in fact the conceptual connection is close. The starting point for both discussions of genealogical trees is an extraordinary all-or-nothing observation: If you look back just a few dozen generations in human history, it’s a reasonable approximation to say that anyone who was alive then is either an ancestor of everyone alive now or else is an ancestor of no one living today—the lineage went extinct. And the probability of extinction for human families is the same curious number encountered in rumor studies; Chang gives the value as 0. 20319, whereas Derrida, Manrubia and Zanette supply a few more decimal places—0.20318787. . .

Krapivsky adds that it’s not altogether suprising for a number like 0.203188 to turn up in diverse contexts, because it derives from a fairly simple mathematical process. The number can be defined as the fixed point of the equation

S = e^{2S-2}

There's no point really, but he takes you on a interesting walk about. His newest column in American Scientist on the information theory and practical application of reversibility in computing is highly entertaining in a comp-geek sort of way. Whether he intends them or not isn't clear, but his articles are full of allusions and double-entendres that I find amusing.

A reversible computer is a better-behaved device, more at home in the universe we live in. As Toffoli wrote in 1982: "Computation—whether by man or by machine—is a physical activity. If we want to compute more, faster, better, more efficiently, and more intelligently, we will have to learn more about nature. In a sense, nature has been continually computing the 'next state' of the universe for billions of years; all we have to do—and, actually, all we can do—is 'hitch a ride' on this huge ongoing computation, and try to discover which parts of it happen to go near to where we want."