21540391 has 4 divisors (see below), whose sum is σ = 21556800. Its totient is φ = 21523984.

The previous prime is 21540383. The next prime is 21540437. The reversal of 21540391 is 19304512.

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 21540391 - 2³ = 21540383 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 (21540341) 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, 6046 + ... + 8923.

It is an arithmetic number, because the mean of its divisors is an integer number (5389200).

Almost surely, 2^21540391 is an apocalyptic number.

21540391 is a deficient number, since it is larger than the sum of its proper divisors (16409).

21540391 is a wasteful number, since it uses less digits than its factorization.

21540391 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 16408.

The product of its (nonzero) digits is 1080, while the sum is 25.

The square root of 21540391 is about 4641.1626776057. The cubic root of 21540391 is about 278.2389083725.

The spelling of 21540391 in words is "twenty-one million, five hundred forty thousand, three hundred ninety-one".