21723781 has 4 divisors (see below), whose sum is σ = 21834252. Its totient is φ = 21613312.

The previous prime is 21723763. The next prime is 21723791. The reversal of 21723781 is 18732712.

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 21538881 + 184900 = 4641^2 + 430^2 .

It is a cyclic number.

It is not a de Polignac number, because 21723781 - 2⁵ = 21723749 is a prime.

It is a super-2 number, since 2×21723781² = 943845321871922, which contains 22 as substring.

It is a Smith number, since the sum of its digits (31) coincides with the sum of the digits of its prime factors. Since it is squarefree, it is also a hoax number.

It is a Duffinian number.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21723701) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (13) of ones.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 54940 + ... + 55333.

It is an arithmetic number, because the mean of its divisors is an integer number (5458563).

Almost surely, 2^21723781 is an apocalyptic number.

It is an amenable number.

21723781 is a deficient number, since it is larger than the sum of its proper divisors (110471).

21723781 is a wasteful number, since it uses less digits than its factorization.

21723781 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 110470.

The product of its digits is 4704, while the sum is 31.

The square root of 21723781 is about 4660.8777070419. The cubic root of 21723781 is about 279.0262989987.

The spelling of 21723781 in words is "twenty-one million, seven hundred twenty-three thousand, seven hundred eighty-one".