21216561 has 8 divisors (see below), whose sum is σ = 29952864. Its totient is φ = 13312320.

The previous prime is 21216553. The next prime is 21216607. The reversal of 21216561 is 16561212.

It is a sphenic number, since it is the product of 3 distinct primes.

It is not a de Polignac number, because 21216561 - 2³ = 21216553 is a prime.

It is a Smith number, since the sum of its digits (24) coincides with the sum of the digits of its prime factors. Since it is squarefree, it is also a hoax number.

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

It is not an unprimeable number, because it can be changed into a prime (21216551) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (13) of ones.

It is a polite number, since it can be written in 7 ways as a sum of consecutive naturals, for example, 207955 + ... + 208056.

It is an arithmetic number, because the mean of its divisors is an integer number (3744108).

Almost surely, 2^21216561 is an apocalyptic number.

It is an amenable number.

21216561 is a deficient number, since it is larger than the sum of its proper divisors (8736303).

21216561 is a wasteful number, since it uses less digits than its factorization.

21216561 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 416031.

The product of its digits is 720, while the sum is 24.

The square root of 21216561 is about 4606.1438318837. The cubic root of 21216561 is about 276.8375497927.

Adding to 21216561 its reverse (16561212), we get a palindrome (37777773).

It can be divided in two parts, 212165 and 61, that added together give a triangular number (212226 = T₆₅₁).

The spelling of 21216561 in words is "twenty-one million, two hundred sixteen thousand, five hundred sixty-one".