21720191 has 4 divisors (see below), whose sum is σ = 21752952. Its totient is φ = 21687432.

The previous prime is 21720137. The next prime is 21720197. The reversal of 21720191 is 19102712.

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

It is a cyclic number.

It is a de Polignac number, because none of the positive numbers 2^k-21720191 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 (21720197) 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, 15365 + ... + 16718.

It is an arithmetic number, because the mean of its divisors is an integer number (5438238).

Almost surely, 2^21720191 is an apocalyptic number.

21720191 is a deficient number, since it is larger than the sum of its proper divisors (32761).

21720191 is an equidigital number, since it uses as much as digits as its factorization.

21720191 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 32760.

The product of its (nonzero) digits is 252, while the sum is 23.

The square root of 21720191 is about 4660.4925705337. The cubic root of 21720191 is about 279.0109278317.

The spelling of 21720191 in words is "twenty-one million, seven hundred twenty thousand, one hundred ninety-one".