21215521 has 4 divisors (see below), whose sum is σ = 21323412. Its totient is φ = 21107632.

The previous prime is 21215503. The next prime is 21215539. The reversal of 21215521 is 12551212.

It is a semiprime because it is the product of two primes.

It is an interprime number because it is at equal distance from previous prime (21215503) and next prime (21215539).

It can be written as a sum of positive squares in 2 ways, for example, as 19927296 + 1288225 = 4464^2 + 1135^2 .

It is a cyclic number.

It is not a de Polignac number, because 21215521 - 2⁵ = 21215489 is a prime.

It is a Duffinian number.

It is not an unprimeable number, because it can be changed into a prime (21215561) 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, 53650 + ... + 54043.

It is an arithmetic number, because the mean of its divisors is an integer number (5330853).

Almost surely, 2^21215521 is an apocalyptic number.

It is an amenable number.

21215521 is a deficient number, since it is larger than the sum of its proper divisors (107891).

21215521 is a wasteful number, since it uses less digits than its factorization.

21215521 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 107890.

The product of its digits is 200, while the sum is 19.

The square root of 21215521 is about 4606.0309378032. The cubic root of 21215521 is about 276.8330263491.

Adding to 21215521 its reverse (12551212), we get a palindrome (33766733).

It can be divided in two parts, 21215 and 521, that added together give a triangular number (21736 = T₂₀₈).

The spelling of 21215521 in words is "twenty-one million, two hundred fifteen thousand, five hundred twenty-one".