21205521 has 6 divisors (see below), whose sum is σ = 30630210. Its totient is φ = 14137008.

The previous prime is 21205453. The next prime is 21205523. The reversal of 21205521 is 12550212.

It can be written as a sum of positive squares in only one way, i.e., 14930496 + 6275025 = 3864^2 + 2505^2 .

It is not a de Polignac number, because 21205521 - 2¹⁰ = 21204497 is a prime.

It is a junction number, because it is equal to n+sod(n) for n = 21205494 and 21205503.

It is not an unprimeable number, because it can be changed into a prime (21205523) by changing a digit.

It is a polite number, since it can be written in 5 ways as a sum of consecutive naturals, for example, 1178076 + ... + 1178093.

It is an arithmetic number, because the mean of its divisors is an integer number (5105035).

Almost surely, 2^21205521 is an apocalyptic number.

It is an amenable number.

21205521 is a deficient number, since it is larger than the sum of its proper divisors (9424689).

21205521 is a wasteful number, since it uses less digits than its factorization.

21205521 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 2356175 (or 2356172 counting only the distinct ones).

The product of its (nonzero) digits is 200, while the sum is 18.

The square root of 21205521 is about 4604.9452765478. The cubic root of 21205521 is about 276.7895241519.

Adding to 21205521 its reverse (12550212), we get a palindrome (33755733).

The spelling of 21205521 in words is "twenty-one million, two hundred five thousand, five hundred twenty-one".