21681431 has 4 divisors (see below), whose sum is σ = 22380864. Its totient is φ = 20982000.

The previous prime is 21681421. The next prime is 21681449. The reversal of 21681431 is 13418612.

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 21681431 - 2¹⁸ = 21419287 is a prime.

It is a super-2 number, since 2×21681431² = 940168900415522, which contains 22 as substring.

It is a Duffinian number.

It is a self number, because there is not a number n which added to its sum of digits gives 21681431.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21681421) 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 in 3 ways as a sum of consecutive naturals, for example, 349670 + ... + 349731.

It is an arithmetic number, because the mean of its divisors is an integer number (5595216).

Almost surely, 2^21681431 is an apocalyptic number.

21681431 is a deficient number, since it is larger than the sum of its proper divisors (699433).

21681431 is an equidigital number, since it uses as much as digits as its factorization.

21681431 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 699432.

The product of its digits is 1152, while the sum is 26.

The square root of 21681431 is about 4656.3323549764. The cubic root of 21681431 is about 278.8448626437.

The spelling of 21681431 in words is "twenty-one million, six hundred eighty-one thousand, four hundred thirty-one".