21729409 has 4 divisors (see below), whose sum is σ = 23400916. Its totient is φ = 20057904.

The previous prime is 21729403. The next prime is 21729427. The reversal of 21729409 is 90492712.

It is a happy number.

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 1882384 + 19847025 = 1372^2 + 4455^2 .

It is a cyclic number.

It is not a de Polignac number, because 21729409 - 2⁵ = 21729377 is a prime.

It is a Duffinian number.

It is not an unprimeable number, because it can be changed into a prime (21729403) 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, 835734 + ... + 835759.

It is an arithmetic number, because the mean of its divisors is an integer number (5850229).

Almost surely, 2^21729409 is an apocalyptic number.

It is an amenable number.

21729409 is a deficient number, since it is larger than the sum of its proper divisors (1671507).

21729409 is a wasteful number, since it uses less digits than its factorization.

21729409 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 1671506.

The product of its (nonzero) digits is 9072, while the sum is 34.

The square root of 21729409 is about 4661.4814168888. The cubic root of 21729409 is about 279.0503927852.

The spelling of 21729409 in words is "twenty-one million, seven hundred twenty-nine thousand, four hundred nine".