21213791 has 4 divisors (see below), whose sum is σ = 21482400. Its totient is φ = 20945184.

The previous prime is 21213781. The next prime is 21213799. The reversal of 21213791 is 19731212.

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 21213791 - 2¹⁸ = 20951647 is a prime.

It is a Duffinian number.

It is a self number, because there is not a number n which added to its sum of digits gives 21213791.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21213799) 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, 134186 + ... + 134343.

It is an arithmetic number, because the mean of its divisors is an integer number (5370600).

Almost surely, 2^21213791 is an apocalyptic number.

21213791 is a deficient number, since it is larger than the sum of its proper divisors (268609).

21213791 is an equidigital number, since it uses as much as digits as its factorization.

21213791 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 268608.

The product of its digits is 756, while the sum is 26.

The square root of 21213791 is about 4605.8431367123. The cubic root of 21213791 is about 276.8255014470.

The spelling of 21213791 in words is "twenty-one million, two hundred thirteen thousand, seven hundred ninety-one".