21149981 has 4 divisors (see below), whose sum is σ = 21496764. Its totient is φ = 20803200.

The previous prime is 21149971. The next prime is 21149987. The reversal of 21149981 is 18994112.

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 2088025 + 19061956 = 1445^2 + 4366^2 .

It is a cyclic number.

It is a de Polignac number, because none of the positive numbers 2^k-21149981 is a prime.

It is a super-2 number, since 2×21149981² = 894643392600722, which contains 22 as substring.

It is a Duffinian number.

It is a Curzon number.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21149987) 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, 173300 + ... + 173421.

It is an arithmetic number, because the mean of its divisors is an integer number (5374191).

Almost surely, 2^21149981 is an apocalyptic number.

It is an amenable number.

21149981 is a deficient number, since it is larger than the sum of its proper divisors (346783).

21149981 is an equidigital number, since it uses as much as digits as its factorization.

21149981 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 346782.

The product of its digits is 5184, while the sum is 35.

The square root of 21149981 is about 4598.9108493207. The cubic root of 21149981 is about 276.5476636960.

The spelling of 21149981 in words is "twenty-one million, one hundred forty-nine thousand, nine hundred eighty-one".