21216979 has 4 divisors (see below), whose sum is σ = 24247984. Its totient is φ = 18185976.

The previous prime is 21216961. The next prime is 21216983. The reversal of 21216979 is 97961212.

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

It is a cyclic number.

It is not a de Polignac number, because 21216979 - 2⁷ = 21216851 is a prime.

It is a Duffinian number.

It is not an unprimeable number, because it can be changed into a prime (21216919) 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, 1515492 + ... + 1515505.

It is an arithmetic number, because the mean of its divisors is an integer number (6061996).

Almost surely, 2^21216979 is an apocalyptic number.

21216979 is a deficient number, since it is larger than the sum of its proper divisors (3031005).

21216979 is an equidigital number, since it uses as much as digits as its factorization.

21216979 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 3031004.

The product of its digits is 13608, while the sum is 37.

The square root of 21216979 is about 4606.1892058403. The cubic root of 21216979 is about 276.8393678275.

The spelling of 21216979 in words is "twenty-one million, two hundred sixteen thousand, nine hundred seventy-nine".