21508997 has 4 divisors (see below), whose sum is σ = 21534240. Its totient is φ = 21483756.

The previous prime is 21508973. The next prime is 21509003. The reversal of 21508997 is 79980512.

It is a semiprime because it is the product of two primes, and also a Blum integer, because the two primes are equal to 3 mod 4.

It is a cyclic number.

It is not a de Polignac number, because 21508997 - 2¹⁸ = 21246853 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 (21508957) 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, 11297 + ... + 13062.

It is an arithmetic number, because the mean of its divisors is an integer number (5383560).

Almost surely, 2^21508997 is an apocalyptic number.

It is an amenable number.

21508997 is a deficient number, since it is larger than the sum of its proper divisors (25243).

21508997 is an equidigital number, since it uses as much as digits as its factorization.

21508997 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 25242.

The product of its (nonzero) digits is 45360, while the sum is 41.

The square root of 21508997 is about 4637.7793177339. The cubic root of 21508997 is about 278.1036697356.

The spelling of 21508997 in words is "twenty-one million, five hundred eight thousand, nine hundred ninety-seven".