21724781 has 6 divisors (see below), whose sum is σ = 23524650. Its totient is φ = 20053488.

The previous prime is 21724757. The next prime is 21724783. The reversal of 21724781 is 18742712.

It is a happy number.

It can be written as a sum of positive squares in 3 ways, for example, as 2941225 + 18783556 = 1715^2 + 4334^2 .

It is not a de Polignac number, because 21724781 - 2¹⁴ = 21708397 is a prime.

It is a super-2 number, since 2×21724781² = 943932218995922, 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 (21724783) by changing a digit.

It is a polite number, since it can be written in 5 ways as a sum of consecutive naturals, for example, 64106 + ... + 64443.

It is an arithmetic number, because the mean of its divisors is an integer number (3920775).

Almost surely, 2^21724781 is an apocalyptic number.

It is an amenable number.

21724781 is a deficient number, since it is larger than the sum of its proper divisors (1799869).

21724781 is a wasteful number, since it uses less digits than its factorization.

21724781 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 128575 (or 128562 counting only the distinct ones).

The product of its digits is 6272, while the sum is 32.

The square root of 21724781 is about 4660.9849817394. The cubic root of 21724781 is about 279.0305803593.

The spelling of 21724781 in words is "twenty-one million, seven hundred twenty-four thousand, seven hundred eighty-one".