21408639 has 12 divisors (see below), whose sum is σ = 33205464. Its totient is φ = 12233424.

The previous prime is 21408623. The next prime is 21408659. The reversal of 21408639 is 93680412.

It is not a de Polignac number, because 21408639 - 2⁴ = 21408623 is a prime.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21408659) by changing a digit.

It is a polite number, since it can be written in 11 ways as a sum of consecutive naturals, for example, 72672 + ... + 72965.

It is an arithmetic number, because the mean of its divisors is an integer number (2767122).

Almost surely, 2^21408639 is an apocalyptic number.

21408639 is a deficient number, since it is larger than the sum of its proper divisors (11796825).

21408639 is a wasteful number, since it uses less digits than its factorization.

21408639 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 145654 (or 145647 counting only the distinct ones).

The product of its (nonzero) digits is 10368, while the sum is 33.

The square root of 21408639 is about 4626.9470496214. The cubic root of 21408639 is about 277.6704642122.

The spelling of 21408639 in words is "twenty-one million, four hundred eight thousand, six hundred thirty-nine".