Just writing "81" is mathematical enough for my taste. After all, that's a number, and numbers are mathematical.
81+0=81
80+1= 81
81 is a whole number whose simplest form is 81.
81 = 8.1 x 101
81/100
81,000,000,000,000
81/10
3
81/100
2.5X102
|PQ|
to write in number form