21684387 has 4 divisors (see below), whose sum is σ = 28912520. Its totient is φ = 14456256.

The previous prime is 21684371. The next prime is 21684409. The reversal of 21684387 is 78348612.

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

It is not a de Polignac number, because 21684387 - 2⁴ = 21684371 is a prime.

It is a Duffinian number.

It is not an unprimeable number, because it can be changed into a prime (21684317) 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, 3614062 + ... + 3614067.

It is an arithmetic number, because the mean of its divisors is an integer number (7228130).

Almost surely, 2^21684387 is an apocalyptic number.

21684387 is a deficient number, since it is larger than the sum of its proper divisors (7228133).

21684387 is an equidigital number, since it uses as much as digits as its factorization.

21684387 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 7228132.

The product of its digits is 64512, while the sum is 39.

The square root of 21684387 is about 4656.6497613628. The cubic root of 21684387 is about 278.8575344389.

It can be divided in two parts, 2168 and 4387, that added together give a triangular number (6555 = T₁₁₄).

The spelling of 21684387 in words is "twenty-one million, six hundred eighty-four thousand, three hundred eighty-seven".