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Cactus patterns

An evening walk for a change, with the sun casting a different light on many of my favorite cactuses. This one has many hidden stories to tell—what caused the change in growth, what governs the pattern of the ridges, what causes the discoloration, who’s been nibbling?

I saw a Mockingbird against the evening sky.

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About Bill McCallum

I was born in Australia and came to the United States to pursue a Ph. D. in mathematics at Harvard University, met my wife, and never went back. I am a professor at the University of Arizona, working in number theory and mathematics education.


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