About 73

“So what does Professor Ono think of seventy-three? “I really like the number seventy-three,” he said. “It is the sixth ‘emirp.’ ” An emirp, he explained, is a prime number that remains prime when its digits are reversed. (Emirp, of course, is “prime” spelled backward.) “Seventy-three and thirty-seven are both prime, so both are emirps.” In an episode of “The Big Bang Theory,” the character Sheldon Lee Cooper says that this is one of several properties that make seventy-three “the best number.” As for the other properties: “It is the twenty-first prime number. Its mirror, thirty-seven, is the twelfth. And its mirror, twenty-one, is the product of multiplying—hang onto your hats—seven and three.” He goes on to add, “In binary, seventy-three is a palindrome: 1001001.”

Seventy-three is also, it turns out, a star number (meaning it can be plotted on a centered hexagram, or Chinese checkers board), the largest minimal primitive root in the first hundred thousand primes (you can find an explanation for that one elsewhere online, if you must), and the smallest number with twelve letters in its name when it’s spelled out. To mathematicians, like Ono, it is more interesting than the average integer.

Seventy-three is also the number of books in the Catholic Bible and the titular number of one of Shakespeare’s most famous sonnets, about old age, which concludes: “This thou perceiv’st, which makes thy love more strong / To love that well which thou must leave ere long.” Perhaps more important to the young people of today, it is the atomic number of tantalum, a rare chemical element used in mobile phones that was named after Tantalus, an antihero in Greek mythology, who suffered hunger and thirst for eternity.”

Golden State and the Mathematical Magic of Seventy-Three

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