Word buffs have always had crossword puzzles, but the only recreation known to mathematicians until the mid-twentieth century was finding novel geometrical proofs for the Pythagorean theorem, one of which is credited to US President James A. Garfield. The construction of his proof is shown in the figure, from which the obvious solution can be derived (at least by my high school geometry teacher!). If you want to cheat, the solution can be found here.

Of course, such proofs become more difficult as time goes on and all the obvious ones have been found. There are at least 370 [1]. A revival in recreational mathematics occurred in 1956 with inception of the Mathematical Games column in Scientific American. This column, which appeared from 1956-1981 was written by Martin Gardner, a veteran science writer who died May 22, 2010, at age 95 [2].

The amazing thing was that Gardner, who was venerated by the mathematician and fellow author, Rudy Rucker, and the physicist and fellow author, Douglas Hofstadter, never took a mathematics course beyond high school. Said Gardner, "There is no better way to learn anything than to write about it!" [2]

One thing that I had in common with Gardner was the idea that mathematics is discovered, not invented. I wrote about this philosophical debate in a previous article (Mathematical Objects, May 09, 2008). This way of looking at mathematics is called the *Platonist* view, and we can count among our fellow Platonists the prominent mathematician, Roger Penrose. One argument that favors discovery is the existence of the constant, pi, the ratio of the circumference to the diameter of a circle. The number, pi, is part of many fundamental equations of physics, which indicates to me that pi has a physical reality independent of an observer.

Gardner's best legacy was legitimizing computers as an important part of mathematics. The proof of the four-color conjecture, a proof done completely by computer, was uniformly derided by professional mathematicians as not being a *real* proof. Gardner, however, wasn't a professional mathematician, so he wasn't bothered by any concept of mathematical purity. One of his most popular articles was about John Conway's Game of Life, a computer excursion into cellular automata. This computer simulation, known to all computer literate people, involves the evolution of a population on a checkerboard array. The individuals of this population are black spots who live alone in separate cells on the white board and obey these simple rules:

- Any person with fewer than two neighbors dies (underpopulation).
- Any person with more than three neighbors dies (overcrowding).
- Any person with two or three neighbors lives on to the next generation (subsistence).
- Any empty cell with exactly three live neighbors generates a new person (birth).