21724981 has 4 divisors (see below), whose sum is σ = 21809772. Its totient is φ = 21640192.

The previous prime is 21724943. The next prime is 21724991. The reversal of 21724981 is 18942712.

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 13256881 + 8468100 = 3641^2 + 2910^2 .

It is a cyclic number.

It is not a de Polignac number, because 21724981 - 2¹¹ = 21722933 is a prime.

It is a super-2 number, since 2×21724981² = 943949598900722, 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 (21724931) 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, 42010 + ... + 42523.

It is an arithmetic number, because the mean of its divisors is an integer number (5452443).

Almost surely, 2^21724981 is an apocalyptic number.

It is an amenable number.

21724981 is a deficient number, since it is larger than the sum of its proper divisors (84791).

21724981 is an equidigital number, since it uses as much as digits as its factorization.

21724981 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 84790.

The product of its digits is 8064, while the sum is 34.

The square root of 21724981 is about 4661.0064363826. The cubic root of 21724981 is about 279.0314366156.

The spelling of 21724981 in words is "twenty-one million, seven hundred twenty-four thousand, nine hundred eighty-one".