21216521 has 4 divisors (see below), whose sum is σ = 22333200. Its totient is φ = 20099844.

The previous prime is 21216497. The next prime is 21216529. The reversal of 21216521 is 12561212.

It is a semiprime because it is the product of two primes, and also a Blum integer, because the two primes are equal to 3 mod 4.

It is a cyclic number.

It is not a de Polignac number, because 21216521 - 2⁶ = 21216457 is a prime.

It is an alternating number because its digits alternate between even and odd.

It is a Duffinian number.

It is a junction number, because it is equal to n+sod(n) for n = 21216493 and 21216502.

It is not an unprimeable number, because it can be changed into a prime (21216529) 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, 558311 + ... + 558348.

It is an arithmetic number, because the mean of its divisors is an integer number (5583300).

Almost surely, 2^21216521 is an apocalyptic number.

It is an amenable number.

21216521 is a deficient number, since it is larger than the sum of its proper divisors (1116679).

21216521 is a wasteful number, since it uses less digits than its factorization.

21216521 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 1116678.

The product of its digits is 240, while the sum is 20.

The square root of 21216521 is about 4606.1394898548. The cubic root of 21216521 is about 276.8373758169.

Adding to 21216521 its reverse (12561212), we get a palindrome (33777733).

The spelling of 21216521 in words is "twenty-one million, two hundred sixteen thousand, five hundred twenty-one".