Numb and Number

Eighty-one is the square of 9 and the fourth power of 3. Like all powers of three, 81 is a perfect totient number. It is a heptagonal number and a centered octagonal number. It is also a tribonacci number, an open meandric number. 81 is the ninth member of the Mian-Chowla sequence.

Unfortunately, Wikipedia doesn't mention this number 81, on our office gate. And those other facts are all a bit too much for me today, as I feel like I have a head full of glue. Mondays, eh?