21125557 has 4 divisors (see below), whose sum is σ = 21696556. Its totient is φ = 20554560.

The previous prime is 21125551. The next prime is 21125603. The reversal of 21125557 is 75552112.

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 12680721 + 8444836 = 3561^2 + 2906^2 .

It is a cyclic number.

It is not a de Polignac number, because 21125557 - 2¹⁵ = 21092789 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 (21125551) 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, 285444 + ... + 285517.

It is an arithmetic number, because the mean of its divisors is an integer number (5424139).

Almost surely, 2^21125557 is an apocalyptic number.

It is an amenable number.

21125557 is a deficient number, since it is larger than the sum of its proper divisors (570999).

21125557 is an equidigital number, since it uses as much as digits as its factorization.

21125557 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 570998.

The product of its digits is 3500, while the sum is 28.

The square root of 21125557 is about 4596.2546709250. The cubic root of 21125557 is about 276.4411702701.

Adding to 21125557 its reverse (75552112), we get a palindrome (96677669).

The spelling of 21125557 in words is "twenty-one million, one hundred twenty-five thousand, five hundred fifty-seven".