21990921 has 16 divisors (see below), whose sum is σ = 30948480. Its totient is φ = 13861760.

The previous prime is 21990907. The next prime is 21990949. The reversal of 21990921 is 12909912.

It is a cyclic number.

It is not a de Polignac number, because 21990921 - 2¹⁹ = 21466633 is a prime.

It is not an unprimeable number, because it can be changed into a prime (21990721) by changing a digit.

It is a polite number, since it can be written in 15 ways as a sum of consecutive naturals, for example, 4351 + ... + 7931.

It is an arithmetic number, because the mean of its divisors is an integer number (1934280).

Almost surely, 2^21990921 is an apocalyptic number.

It is an amenable number.

21990921 is a deficient number, since it is larger than the sum of its proper divisors (8957559).

21990921 is a wasteful number, since it uses less digits than its factorization.

21990921 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 3696.

The product of its (nonzero) digits is 2916, while the sum is 33.

The square root of 21990921 is about 4689.4478353000. The cubic root of 21990921 is about 280.1653827392.

The spelling of 21990921 in words is "twenty-one million, nine hundred ninety thousand, nine hundred twenty-one".