21557993 has 4 divisors (see below), whose sum is σ = 21567300. Its totient is φ = 21548688.

The previous prime is 21557983. The next prime is 21558001. The reversal of 21557993 is 39975512.

It is a semiprime because it is the product of two primes, and also a brilliant number, because the two primes have the same length.

It can be written as a sum of positive squares in 2 ways, for example, as 8111104 + 13446889 = 2848^2 + 3667^2 .

It is a cyclic number.

It is not a de Polignac number, because 21557993 - 2⁸ = 21557737 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 21557993.

It is not an unprimeable number, because it can be changed into a prime (21557953) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (13) of ones.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 1871 + ... + 6827.

It is an arithmetic number, because the mean of its divisors is an integer number (5391825).

Almost surely, 2^21557993 is an apocalyptic number.

It is an amenable number.

21557993 is a deficient number, since it is larger than the sum of its proper divisors (9307).

21557993 is an equidigital number, since it uses as much as digits as its factorization.

21557993 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 9306.

The product of its digits is 85050, while the sum is 41.

The square root of 21557993 is about 4643.0585824433. The cubic root of 21557993 is about 278.3146765396.

The spelling of 21557993 in words is "twenty-one million, five hundred fifty-seven thousand, nine hundred ninety-three".