21205399 has 2 divisors, whose sum is σ = 21205400. Its totient is φ = 21205398.

The previous prime is 21205397. The next prime is 21205447. The reversal of 21205399 is 99350212.

It is a weak prime.

It is a cyclic number.

It is not a de Polignac number, because 21205399 - 2¹ = 21205397 is a prime.

Together with 21205397, it forms a pair of twin primes.

It is a self number, because there is not a number n which added to its sum of digits gives 21205399.

It is a congruent number.

It is not a weakly prime, because it can be changed into another prime (21205391) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 10602699 + 10602700.

It is an arithmetic number, because the mean of its divisors is an integer number (10602700).

Almost surely, 2^21205399 is an apocalyptic number.

21205399 is a deficient number, since it is larger than the sum of its proper divisors (1).

21205399 is an equidigital number, since it uses as much as digits as its factorization.

21205399 is an evil number, because the sum of its binary digits is even.

The product of its (nonzero) digits is 4860, while the sum is 31.

The square root of 21205399 is about 4604.9320299001. The cubic root of 21205399 is about 276.7889933407.

The spelling of 21205399 in words is "twenty-one million, two hundred five thousand, three hundred ninety-nine".