• 91 can be written using four 4's:
It is a happy number.
91 is nontrivially palindromic in base 3, base 9 and base 12.
91 is an esthetic number in base 3 and base 14, because in such bases its adjacent digits differ by 1.
It is a semiprime because it is the product of two primes.
It is a 3-Lehmer number, since φ(91) divides (91-1)3.
It is a cyclic number.
It is the 10-th Hogben number.
It is a deceptive number, since it divides R90.
91 is an undulating number in base 3.
91 is a nontrivial repdigit in base 9 and base 12.
It is a plaindrome in base 4, base 8, base 9, base 12, base 14 and base 16.
It is a nialpdrome in base 5, base 9, base 10, base 11, base 12, base 13 and base 15.
It is a zygodrome in base 9 and base 12.
It is a panconsummate number.
It is an upside-down number.
91 is a wasteful number, since it uses less digits than its factorization.
91 is an odious number, because the sum of its binary digits is odd.
The sum of its prime factors is 20.
The square root of 91 is about 9.5393920142. The cubic root of 91 is about 4.4979414453.
Adding to 91 its sum of digits (10), we get a palindrome (101).
Subtracting from 91 its sum of digits (10), we obtain a 4-th power (81 = 34).