21155521 has 4 divisors (see below), whose sum is σ = 21342852. Its totient is φ = 20968192.

The previous prime is 21155513. The next prime is 21155543. The reversal of 21155521 is 12555112.

It is a happy number.

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 6225025 + 14930496 = 2495^2 + 3864^2 .

It is a cyclic number.

It is not a de Polignac number, because 21155521 - 2³ = 21155513 is a prime.

It is a Duffinian number.

It is a junction number, because it is equal to n+sod(n) for n = 21155492 and 21155501.

It is not an unprimeable number, because it can be changed into a prime (21155501) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (11) of ones.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 93496 + ... + 93721.

It is an arithmetic number, because the mean of its divisors is an integer number (5335713).

Almost surely, 2^21155521 is an apocalyptic number.

It is an amenable number.

21155521 is a deficient number, since it is larger than the sum of its proper divisors (187331).

21155521 is a wasteful number, since it uses less digits than its factorization.

21155521 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 187330.

The product of its digits is 500, while the sum is 22.

The square root of 21155521 is about 4599.5131264081. The cubic root of 21155521 is about 276.5718077730.

The spelling of 21155521 in words is "twenty-one million, one hundred fifty-five thousand, five hundred twenty-one".