21724381 has 4 divisors (see below), whose sum is σ = 24827872. Its totient is φ = 18620892.

The previous prime is 21724379. The next prime is 21724399. The reversal of 21724381 is 18342712.

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 not a de Polignac number, because 21724381 - 2¹ = 21724379 is a prime.

It is a super-2 number, since 2×21724381² = 943897459666322, 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 (21724321) 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, 1551735 + ... + 1551748.

It is an arithmetic number, because the mean of its divisors is an integer number (6206968).

Almost surely, 2^21724381 is an apocalyptic number.

It is an amenable number.

21724381 is a deficient number, since it is larger than the sum of its proper divisors (3103491).

21724381 is an equidigital number, since it uses as much as digits as its factorization.

21724381 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 3103490.

The product of its digits is 2688, while the sum is 28.

The square root of 21724381 is about 4660.9420721567. The cubic root of 21724381 is about 279.0288678308.

The spelling of 21724381 in words is "twenty-one million, seven hundred twenty-four thousand, three hundred eighty-one".