21506231 has 4 divisors (see below), whose sum is σ = 21574392. Its totient is φ = 21438072.

The previous prime is 21506209. The next prime is 21506257. The reversal of 21506231 is 13260512.

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 21506231 - 2⁶ = 21506167 is a prime.

It is a super-2 number, since 2×21506231² = 925035943650722, 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 (21506201) 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, 33605 + ... + 34238.

It is an arithmetic number, because the mean of its divisors is an integer number (5393598).

Almost surely, 2^21506231 is an apocalyptic number.

21506231 is a deficient number, since it is larger than the sum of its proper divisors (68161).

21506231 is an equidigital number, since it uses as much as digits as its factorization.

21506231 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 68160.

The product of its (nonzero) digits is 360, while the sum is 20.

The square root of 21506231 is about 4637.4811050828. The cubic root of 21506231 is about 278.0917480930.

Adding to 21506231 its reverse (13260512), we get a palindrome (34766743).

The spelling of 21506231 in words is "twenty-one million, five hundred six thousand, two hundred thirty-one".