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Math Is Beautiful

81 is the only number* that is the square of the sum of its digits: (8 + 1)^2 = 81.

And 18 is the only number that’s twice the sum of its digits: (8 + 1) x 2 = 18.

That is all.

*Well, except for 1, I suppose. But that feels like cheating. (Oh—and if exhaustive lists of the properties of specific numbers is your jam, Robert Munafo has you covered.)



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