21408531 has 4 divisors (see below), whose sum is σ = 28544712. Its totient is φ = 14272352.

The previous prime is 21408529. The next prime is 21408557. The reversal of 21408531 is 13580412.

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 21408531 - 2¹ = 21408529 is a prime.

It is a super-2 number, since 2×21408531² = 916650399155922, which contains 22 as substring.

It is not an unprimeable number, because it can be changed into a prime (21408511) by changing a digit.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 3568086 + ... + 3568091.

It is an arithmetic number, because the mean of its divisors is an integer number (7136178).

Almost surely, 2^21408531 is an apocalyptic number.

21408531 is a deficient number, since it is larger than the sum of its proper divisors (7136181).

21408531 is an equidigital number, since it uses as much as digits as its factorization.

21408531 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 7136180.

The product of its (nonzero) digits is 960, while the sum is 24.

The square root of 21408531 is about 4626.9353788442. The cubic root of 21408531 is about 277.6699972907.

Adding to 21408531 its reverse (13580412), we get a palindrome (34988943).

The spelling of 21408531 in words is "twenty-one million, four hundred eight thousand, five hundred thirty-one".