91221815 has 4 divisors (see below), whose sum is σ = 109466184. Its totient is φ = 72977448.

The previous prime is 91221787. The next prime is 91221817. The reversal of 91221815 is 51812219.

It is a semiprime because it is the product of two primes, and also an emirpimes, since its reverse is a distinct semiprime: 51812219 = 271 ⋅191189.

It is a cyclic number.

It is not a de Polignac number, because 91221815 - 2⁶ = 91221751 is a prime.

It is a Duffinian number.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (91221817) by changing a digit.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 9122177 + ... + 9122186.

It is an arithmetic number, because the mean of its divisors is an integer number (27366546).

Almost surely, 2^91221815 is an apocalyptic number.

91221815 is a deficient number, since it is larger than the sum of its proper divisors (18244369).

91221815 is a wasteful number, since it uses less digits than its factorization.

91221815 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 18244368.

The product of its digits is 1440, while the sum is 29.

The square root of 91221815 is about 9551.0112030088. The cubic root of 91221815 is about 450.1593098493.

The spelling of 91221815 in words is "ninety-one million, two hundred twenty-one thousand, eight hundred fifteen".