81

81 = 3⁴, a perfect fourth power.

81 is the only integer other than 1 that is the square of the sum of its digits.

81 is a heptagonal number.

81 is a centered octagonal number.

81 = 1 + 3 + 5 + 7 + 9 + 11 + 13 + 15 + 17 (the first nine odd numbers).

The chemical element thallium has the atomic number 81.

Source: Number Gossip