21385981 has 4 divisors (see below), whose sum is σ = 21687264. Its totient is φ = 21084700.

The previous prime is 21385967. The next prime is 21385999. The reversal of 21385981 is 18958312.

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-21385981 is a prime.

It is a super-2 number, since 2×21385981² = 914720366664722, 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 21385981.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21385951) 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, 150535 + ... + 150676.

It is an arithmetic number, because the mean of its divisors is an integer number (5421816).

Almost surely, 2^21385981 is an apocalyptic number.

It is an amenable number.

21385981 is a deficient number, since it is larger than the sum of its proper divisors (301283).

21385981 is an equidigital number, since it uses as much as digits as its factorization.

21385981 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 301282.

The product of its digits is 17280, while the sum is 37.

The square root of 21385981 is about 4624.4979186934. The cubic root of 21385981 is about 277.5724714000.

It can be divided in two parts, 21385 and 981, that added together give a triangular number (22366 = T₂₁₁).

The spelling of 21385981 in words is "twenty-one million, three hundred eighty-five thousand, nine hundred eighty-one".