21684491 has 4 divisors (see below), whose sum is σ = 22825800. Its totient is φ = 20543184.

The previous prime is 21684461. The next prime is 21684499. The reversal of 21684491 is 19448612.

It is a happy number.

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 21684491 - 2¹⁴ = 21668107 is a prime.

It is a Duffinian number.

It is not an unprimeable number, because it can be changed into a prime (21684499) 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 3 ways as a sum of consecutive naturals, for example, 570626 + ... + 570663.

It is an arithmetic number, because the mean of its divisors is an integer number (5706450).

Almost surely, 2^21684491 is an apocalyptic number.

21684491 is a deficient number, since it is larger than the sum of its proper divisors (1141309).

21684491 is a wasteful number, since it uses less digits than its factorization.

21684491 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 1141308.

The product of its digits is 13824, while the sum is 35.

The square root of 21684491 is about 4656.6609281759. The cubic root of 21684491 is about 278.8579802456.

The spelling of 21684491 in words is "twenty-one million, six hundred eighty-four thousand, four hundred ninety-one".