21497981 has 4 divisors (see below), whose sum is σ = 22022364. Its totient is φ = 20973600.

The previous prime is 21497977. The next prime is 21498041. The reversal of 21497981 is 18979412.

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 749956 + 20748025 = 866^2 + 4555^2 .

It is a cyclic number.

It is not a de Polignac number, because 21497981 - 2² = 21497977 is a prime.

It is a super-2 number, since 2×21497981² = 924326374152722, which contains 22 as substring.

It is a Harshad number since it is a multiple of its sum of digits (41), and also a Moran number because the ratio is a prime number: 524341 = 21497981 / (2 + 1 + 4 + 9 + 7 + 9 + 8 + 1).

It is a Duffinian number.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21497921) 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, 262130 + ... + 262211.

It is an arithmetic number, because the mean of its divisors is an integer number (5505591).

Almost surely, 2^21497981 is an apocalyptic number.

It is an amenable number.

21497981 is a deficient number, since it is larger than the sum of its proper divisors (524383).

21497981 is an equidigital number, since it uses as much as digits as its factorization.

21497981 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 524382.

The product of its digits is 36288, while the sum is 41.

The square root of 21497981 is about 4636.5915282673. The cubic root of 21497981 is about 278.0561839759.

Subtracting from 21497981 its reverse (18979412), we obtain a square (2518569 = 1587²).

The spelling of 21497981 in words is "twenty-one million, four hundred ninety-seven thousand, nine hundred eighty-one".