21493981 has 4 divisors (see below), whose sum is σ = 21512700. Its totient is φ = 21475264.

The previous prime is 21493957. The next prime is 21493999. The reversal of 21493981 is 18939412.

It is a semiprime because it is the product of two primes.

It can be written as a sum of positive squares in 2 ways, for example, as 9865881 + 11628100 = 3141^2 + 3410^2 .

It is a cyclic number.

It is not a de Polignac number, because 21493981 - 2⁷ = 21493853 is a prime.

It is a super-2 number, since 2×21493981² = 923982438456722, 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 21493981.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21493951) 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, 7516 + ... + 9973.

It is an arithmetic number, because the mean of its divisors is an integer number (5378175).

Almost surely, 2^21493981 is an apocalyptic number.

It is an amenable number.

21493981 is a deficient number, since it is larger than the sum of its proper divisors (18719).

21493981 is a wasteful number, since it uses less digits than its factorization.

21493981 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 18718.

The product of its digits is 15552, while the sum is 37.

The square root of 21493981 is about 4636.1601568539. The cubic root of 21493981 is about 278.0389374924.

The spelling of 21493981 in words is "twenty-one million, four hundred ninety-three thousand, nine hundred eighty-one".