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The number pi is found everywhere. His involvement with circles is well known, but it also crops up in other settings. For example, the series 1/1 2 + 1/2 2 + 1/3 2 + 1/4 2 + 1/5 2 … = 1 + 1/4 + 1/9 + 1/16 + 1/25 .. . gets closer and closer to the value π2 / 6 = 1.645... as more and more terms are added. Flipping this fraction around, we get 6/π2, which is equal to the probability that two numbers, provided they are large enough, are prime—that is, that they have no common factors other than 1.
