21781493 has 8 divisors (see below), whose sum is σ = 22538712. Its totient is φ = 21033216.

The previous prime is 21781481. The next prime is 21781511. The reversal of 21781493 is 39418712.

It can be written as a sum of positive squares in 4 ways, for example, as 8655364 + 13126129 = 2942^2 + 3623^2 .

It is a sphenic number, since it is the product of 3 distinct primes.

It is a cyclic number.

It is not a de Polignac number, because 21781493 - 2⁴ = 21781477 is a prime.

It is a Duffinian number.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21781423) by changing a digit.

It is a polite number, since it can be written in 7 ways as a sum of consecutive naturals, for example, 2921 + ... + 7217.

It is an arithmetic number, because the mean of its divisors is an integer number (2817339).

Almost surely, 2^21781493 is an apocalyptic number.

It is an amenable number.

21781493 is a deficient number, since it is larger than the sum of its proper divisors (757219).

21781493 is a wasteful number, since it uses less digits than its factorization.

21781493 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 4471.

The product of its digits is 12096, while the sum is 35.

The square root of 21781493 is about 4667.0647092150. The cubic root of 21781493 is about 279.2731701847.

The spelling of 21781493 in words is "twenty-one million, seven hundred eighty-one thousand, four hundred ninety-three".