21909397 has 4 divisors (see below), whose sum is σ = 22155660. Its totient is φ = 21663136.

The previous prime is 21909389. The next prime is 21909401. The reversal of 21909397 is 79390912.

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

It can be written as a sum of positive squares in 2 ways, for example, as 9979281 + 11930116 = 3159^2 + 3454^2 .

It is a cyclic number.

It is not a de Polignac number, because 21909397 - 2³ = 21909389 is a prime.

It is a Smith number, since the sum of its digits (40) coincides with the sum of the digits of its prime factors. Since it is squarefree, it is also a hoax number.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21909347) 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, 122998 + ... + 123175.

It is an arithmetic number, because the mean of its divisors is an integer number (5538915).

Almost surely, 2^21909397 is an apocalyptic number.

It is an amenable number.

21909397 is a deficient number, since it is larger than the sum of its proper divisors (246263).

21909397 is an equidigital number, since it uses as much as digits as its factorization.

21909397 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 246262.

The product of its (nonzero) digits is 30618, while the sum is 40.

The square root of 21909397 is about 4680.7474830416. The cubic root of 21909397 is about 279.8187474910.

The spelling of 21909397 in words is "twenty-one million, nine hundred nine thousand, three hundred ninety-seven".