21681391 has 4 divisors (see below), whose sum is σ = 21700000. Its totient is φ = 21662784.

The previous prime is 21681389. The next prime is 21681421. The reversal of 21681391 is 19318612.

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 21681391 - 2¹ = 21681389 is a prime.

It is a Duffinian number.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21681301) by changing a digit.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 7431 + ... + 9928.

It is an arithmetic number, because the mean of its divisors is an integer number (5425000).

Almost surely, 2^21681391 is an apocalyptic number.

21681391 is a deficient number, since it is larger than the sum of its proper divisors (18609).

21681391 is a wasteful number, since it uses less digits than its factorization.

21681391 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 18608.

The product of its digits is 2592, while the sum is 31.

The square root of 21681391 is about 4656.3280597484. The cubic root of 21681391 is about 278.8446911636.

The spelling of 21681391 in words is "twenty-one million, six hundred eighty-one thousand, three hundred ninety-one".