21720091 has 4 divisors (see below), whose sum is σ = 21755800. Its totient is φ = 21684384.

The previous prime is 21720053. The next prime is 21720109. The reversal of 21720091 is 19002712.

It is a semiprime because it is the product of two primes.

It is a cyclic number.

It is not a de Polignac number, because 21720091 - 2¹¹ = 21718043 is a prime.

It is a Duffinian number.

It is not an unprimeable number, because it can be changed into a prime (21720991) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (13) of ones.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 16926 + ... + 18163.

It is an arithmetic number, because the mean of its divisors is an integer number (5438950).

Almost surely, 2^21720091 is an apocalyptic number.

21720091 is a deficient number, since it is larger than the sum of its proper divisors (35709).

21720091 is an equidigital number, since it uses as much as digits as its factorization.

21720091 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 35708.

The product of its (nonzero) digits is 252, while the sum is 22.

The square root of 21720091 is about 4660.4818420417. The cubic root of 21720091 is about 279.0104996413.

The spelling of 21720091 in words is "twenty-one million, seven hundred twenty thousand, ninety-one".