21205231 has 2 divisors, whose sum is σ = 21205232. Its totient is φ = 21205230.

The previous prime is 21205213. The next prime is 21205237. The reversal of 21205231 is 13250212.

Together with previous prime (21205213) it forms an Ormiston pair, because they use the same digits, order apart.

It is a strong prime.

It is a cyclic number.

It is not a de Polignac number, because 21205231 - 2¹⁹ = 20680943 is a prime.

It is a super-2 number, since 2×21205231² = 899323643526722, which contains 22 as substring.

It is a congruent number.

It is not a weakly prime, because it can be changed into another prime (21205237) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (13) of ones.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 10602615 + 10602616.

It is an arithmetic number, because the mean of its divisors is an integer number (10602616).

Almost surely, 2^21205231 is an apocalyptic number.

21205231 is a deficient number, since it is larger than the sum of its proper divisors (1).

21205231 is an equidigital number, since it uses as much as digits as its factorization.

21205231 is an odious number, because the sum of its binary digits is odd.

The product of its (nonzero) digits is 120, while the sum is 16.

The square root of 21205231 is about 4604.9137885524. The cubic root of 21205231 is about 276.7882623842.

Adding to 21205231 its reverse (13250212), we get a palindrome (34455443).

The spelling of 21205231 in words is "twenty-one million, two hundred five thousand, two hundred thirty-one".