21581431 has 2 divisors, whose sum is σ = 21581432. Its totient is φ = 21581430.

The previous prime is 21581389. The next prime is 21581447. The reversal of 21581431 is 13418512.

It is a strong prime.

It is a cyclic number.

It is not a de Polignac number, because 21581431 - 2⁹ = 21580919 is a prime.

It is a super-2 number, since 2×21581431² = 931516328015522, which contains 22 as substring.

It is a junction number, because it is equal to n+sod(n) for n = 21581396 and 21581405.

It is a congruent number.

It is not a weakly prime, because it can be changed into another prime (21581731) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 10790715 + 10790716.

It is an arithmetic number, because the mean of its divisors is an integer number (10790716).

Almost surely, 2^21581431 is an apocalyptic number.

21581431 is a deficient number, since it is larger than the sum of its proper divisors (1).

21581431 is an equidigital number, since it uses as much as digits as its factorization.

21581431 is an evil number, because the sum of its binary digits is even.

The product of its digits is 960, while the sum is 25.

The square root of 21581431 is about 4645.5818795927. The cubic root of 21581431 is about 278.4155018934.

Adding to 21581431 its reverse (13418512), we get a palindrome (34999943).

The spelling of 21581431 in words is "twenty-one million, five hundred eighty-one thousand, four hundred thirty-one".