• 91 can be written using four 4's:

91 has 4 divisors (see below), whose sum is σ = 112. Its totient is φ = 72.

The previous prime is 89. The next prime is 97. The reversal of 91 is 19.

91 = 1^{2} + 2^{2} + ... + 6^{2}.

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.

91 is a nontrivial binomial coefficient, being equal to C(14, 2).

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 not a de Polignac number, because 91 - 2^{1} = 89 is a prime.

It is the 10-th Hogben number.

It is a deceptive number, since it divides R_{90}.

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 a nontrivial repunit in base 9.

It is a pernicious number, because its binary representation contains a prime number (5) of ones.

It is an upside-down number.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 1 + ... + 13.

It is an arithmetic number, because the mean of its divisors is an integer number (28).

91 is the 13-th triangular number and also the 7-th hexagonal number.

91 is the 6-th hex number and also the 5-th centered nonagonal number.

91 is a deficient number, since it is larger than the sum of its proper divisors (21).

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 product of its digits is 9, while the sum is 10.

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 = 3^{4}).

The spelling of 91 in words is "ninety-one", and thus it is an aban number and an uban number.

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