21405231 has 6 divisors (see below), whose sum is σ = 30918680. Its totient is φ = 14270148.

The previous prime is 21405187. The next prime is 21405247. The reversal of 21405231 is 13250412.

21405231 is a `hidden beast` number, since 2 + 140 + 523 + 1 = 666.

It is not a de Polignac number, because 21405231 - 2¹⁷ = 21274159 is a prime.

It is a super-2 number, since 2×21405231² = 916367828326722, which contains 22 as substring.

It is a Duffinian number.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21405931) by changing a digit.

It is a polite number, since it can be written in 5 ways as a sum of consecutive naturals, for example, 1189171 + ... + 1189188.

Almost surely, 2^21405231 is an apocalyptic number.

21405231 is a deficient number, since it is larger than the sum of its proper divisors (9513449).

21405231 is a wasteful number, since it uses less digits than its factorization.

21405231 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 2378365 (or 2378362 counting only the distinct ones).

The product of its (nonzero) digits is 240, while the sum is 18.

The square root of 21405231 is about 4626.5787575702. The cubic root of 21405231 is about 277.6557294882.

Adding to 21405231 its reverse (13250412), we get a palindrome (34655643).

The spelling of 21405231 in words is "twenty-one million, four hundred five thousand, two hundred thirty-one".