21991921 has 8 divisors (see below), whose sum is σ = 25228544. Its totient is φ = 18779040.

The previous prime is 21991919. The next prime is 21991939. The reversal of 21991921 is 12919912.

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

It is not a de Polignac number, because 21991921 - 2¹ = 21991919 is a prime.

It is a Duffinian number.

It is not an unprimeable number, because it can be changed into a prime (21991951) by changing a digit.

It is a polite number, since it can be written in 7 ways as a sum of consecutive naturals, for example, 3900 + ... + 7693.

It is an arithmetic number, because the mean of its divisors is an integer number (3153568).

Almost surely, 2^21991921 is an apocalyptic number.

It is an amenable number.

21991921 is a deficient number, since it is larger than the sum of its proper divisors (3236623).

21991921 is a wasteful number, since it uses less digits than its factorization.

21991921 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 11871.

The product of its digits is 2916, while the sum is 34.

The square root of 21991921 is about 4689.5544564489. The cubic root of 21991921 is about 280.1696293574.

The spelling of 21991921 in words is "twenty-one million, nine hundred ninety-one thousand, nine hundred twenty-one".