21872921 has 12 divisors (see below), whose sum is σ = 25324992. Its totient is φ = 18505584.

The previous prime is 21872891. The next prime is 21872923. The reversal of 21872921 is 12927812.

It is a happy number.

It is not a de Polignac number, because 21872921 - 2¹⁴ = 21856537 is a prime.

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 (21872923) 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 11 ways as a sum of consecutive naturals, for example, 83036 + ... + 83298.

It is an arithmetic number, because the mean of its divisors is an integer number (2110416).

Almost surely, 2^21872921 is an apocalyptic number.

It is an amenable number.

21872921 is a deficient number, since it is larger than the sum of its proper divisors (3452071).

21872921 is an equidigital number, since it uses as much as digits as its factorization.

21872921 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 488 (or 379 counting only the distinct ones).

The product of its digits is 4032, while the sum is 32.

The square root of 21872921 is about 4676.8494737376. The cubic root of 21872921 is about 279.6633752232.

The spelling of 21872921 in words is "twenty-one million, eight hundred seventy-two thousand, nine hundred twenty-one".