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.)