21685981 has 4 divisors (see below), whose sum is σ = 22053600. Its totient is φ = 21318364.

The previous prime is 21685967. The next prime is 21686023. The reversal of 21685981 is 18958612.

It is a semiprime because it is the product of two primes, and also a Blum integer, because the two primes are equal to 3 mod 4.

It is a cyclic number.

It is a de Polignac number, because none of the positive numbers 2^k-21685981 is a prime.

It is a super-2 number, since 2×21685981² = 940563543864722, 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 (21688981) 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, 183721 + ... + 183838.

It is an arithmetic number, because the mean of its divisors is an integer number (5513400).

Almost surely, 2^21685981 is an apocalyptic number.

It is an amenable number.

21685981 is a deficient number, since it is larger than the sum of its proper divisors (367619).

21685981 is an equidigital number, since it uses as much as digits as its factorization.

21685981 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 367618.

The product of its digits is 34560, while the sum is 40.

The square root of 21685981 is about 4656.8209113085. The cubic root of 21685981 is about 278.8643671279.

The spelling of 21685981 in words is "twenty-one million, six hundred eighty-five thousand, nine hundred eighty-one".